GURPS does morale rolls, too, as a Reaction Roll. There have been times I've lamented the lack of a quick-and-easy Morale stat - something I could roll against. Succeed, and they stand and fight. Fail, and then flee or surrender. I've tried to make one, and learned a lot in the process about why I like Reaction Rolls for morale.
This post are some of my thoughts of how to deal with this in GURPS.
Morale as Reaction Roll
Using a reaction roll for morale checks can be odd. Part of it is a perspective issue. A morale check is a reaction roll by NPCs in combat. The worse they feel about the PCs and the better they feel about fighting them, the less likely they are to break off combat. The better they feel about the PCs, and the worse they feel about fighting them, the more likely they'll flee or surrender (or, logically, negotiate, although it doesn't mention that specifically). In other worse, you're not checking the NPC's "morale" per se, you're checking how they feel about continuing to fight the PCs. "How you like me now?"
Likability Matters - This makes a lot of sense in that positive reaction modifiers make combat less likely - Charisma, reputation for good things, etc. Also, negative reaction modifiers make combat more likely - bad reputation, racial discord issues, intolerance, etc. They also make you more or less committed to the fight. You're therefore less likely to keep fighting when it's going poorly and you are against Charismatic folks with a reputation for chivalry and honor. That makes sense to me.
Plus the situational modifiers skew the view of combat - a position of strength for the PCs makes combat less likely, a position of weakness more likely, a lack of communication and/or invading someone's territory makes combat more likely.
Parallels the Social Skill System - Intimidation, Diplomacy, Fast-Talk, etc. all can force a "Good" or better reaction, or fail and cause a bad one (Diplomacy has an exception right here). So a "Good" reaction meaning no combat and a "Good" reaction meaning the foes break off the fight or give up? Nice parallel.
PCs don't roll Reaction Rolls - this is a hidden bit here. PCs are subject to all of the rules NPCs are, except for a very few. One of them is that social skills affect them differently (give penalties but don't dictate results) and they don't roll reaction rolls.
It potentially drives fight-and-negotiate strategies - Combat is reaction rolls by other means. Your ultimate goal - win the fight - is served by making the foes less able to resist your general negotiating demands.
Put it another way, you can shift an NPC's willingness to let you have what you want by fighting. This isn't crazy. There were instances in the 16th century of Sir Francis Drake storming ashore in the Spanish Main, shooting up the Spanish garrison for a bit, and then demanding they trade with him. Drake and his men even paid for the privilege - as bribe to the local head honcho to accept the demand. Why not just finish them off and loot? Because his goal was to open up trade for England, not loot the place silly (that came later, during a more hostile Anglo-Spanish relationship.) Why the bribe? Because it's both a carrot (you get stuff if you cooperate) and a stick (we'll keep successfully fighting you if you don't) - it shows a clear distinction between cooperation and non-cooperation.
Tying Morale ("Do I want to keep fighting these people?") to the same rules used for negotiation ("Do I want to give these people what they want?") means these are linked activities. Making a Morale check separate doesn't make this impossible, by any means, but it makes the link less plain.
One of the big issues is really how it feels - it feels weird to roll to see how much the bad guys like fighting you. Or like you in a fight. That's a non-trivial issue for a lot of folks.
Morale as Stat Check
Why not make it a stat check (and therefore, either a stat or a pseudo-stat?) Henchman do this, pretty much, with Loyalty checks?
Simple - It really would be simple, once implemented. Roll and succeed, roll and fail, follow the rules associated with those successes and failures.
Direct - You can apply only the modifiers that matter - situational modifiers and specific reaction modifiers for bravery (Fearlessness would help, obviously, and Cowardice hinder) and situations (outnumbers by X to 1 = -1, 2X to 1 = -2, etc. or just "by a little" or "by a lot.")
There are issues, though:
Which of the opponent's reaction modifiers matter? In other words, if a foe has Charisma +1 but he's also a dwarf and you're a dwarf-hating orc (Intolerance, gives -3), do you roll at a net +2 (-3 and +1) or +4 (-3 and +1), or a -3 (Charisma doesn't matter), or what? Is that purely a reaction but it doesn't affect your bravery or willingness to fight just a little longer against the hated dwarves? Does it matter if that dwarf has a reputation for treating orcs with honor despite his racial handicap? Are you more likely to surrender or less?
It's a question that needs addressing. It's a question of "What is Morale?" - is it bravery for purposing of continuing to fight (pass it and you can fight) or a merged bravery and interest in fighting? If it's the former, it doesn't matter who you are fighting except for relative strength, if that.
Which monsters don't roll? - Making it a stat also means some monsters shouldn't need to roll, at all, but will have a stat. Is this a free benefit or an Immunity? If it's free, who gets it? Automatons, for sure. Low-IQ bugs and slimes, perhaps? Do demons have morale?
PCs roll, too? - This is a hidden downside. If you're making it a stat check, the PCs should be affected, too. Probably like Social Skills - if you fail the roll, you are penalized for actions logically affected by it.
Unless you want to make an attribute that NPCs roll against but PCs do not, of course. But that puts NPCs in a weird bind. A "Morale" stat should be penalized for Cowardice, but Cowardice already has a self-control roll. Do they need to roll both, but PCs with Cowardice only roll once? Does one replace the other? What about Loyalty - do they use the higher or Morale or Loyalty, or the lower?
What I do
The reasons above probably don't seem balanced. They're probably not. I sat down and tried to write a moral check system for DF, and pretty much ended up saying, nah, Reaction Rolls. As odd as that can seem on the surface, I find it's not that difficult. It took some note-taking and thinking to wrap my head around what I need to know. But I like how it plays out. Warlike races fighting their racial foes who are known for not taking prisoners tend to roll in the "keep fighting!" range. Neutral folks fighting people they don't have any specific grievance against tend to split 50/50. And all those "free" disads ("I have Odious Personal Habit, Social Stigma, Intolerance of everyone, and Bloodlust and a tendency to shoot down fleeing foes!") add up quickly as real problems.
So in the end I stayed with Reaction Rolls.
* Just kidding. Of course they always were when we played as kids.**
** And I've mentioned how the adults I game with practice "kill them until then run, then run them down" as a general approach to combat.