This spawned from a Google+ discussion. The original poster put up a funny image (an old one, but still a good one) and we rules guys turned it into a discussion of rules and pricing of Shield Wall Training. I had a few thoughts, as usual.
Shield Wall Training - what's it give you?
It provides two benefits.
1) You can Block for someone adjacent to you on your shield side.
2) You can attack while wielding a Large shield without the usual -2 to hit.
If you've got a Large shield, it's a really great deal. You net a +1 DB over a Medium shield (albeit for a lot of additional weight), and you don't suffer a -2 to attack with your weapon hand. Hurrah!
Even if you don't have a Large shield, it's still useful if you have a pretty good block and a friend you want to protect. If you want to protect people behind you, you need another perk (Sacrificial Block, which is in DF15).
But still, a pretty good deal at 1 point. Should it be more? Well, let's try to price it as a Technique.
First, what's it give you?
You can Block for another person, if they're next to you. That's pretty narrow but it's still something other people can't do. So that's -1 to the default.
You negate the -2 to attack while holding a Large shield. This is tricky to price - Shield is the skill you use, but it's not a -2 to shield, it's a -2 to attack with that skill or another. It's hard to buy off a blanket penalty to an open-ended set of skills with one technique. If you look at Cavalry Training, for example, you buy off the penalty for striking from horseback, but you don't buy it off overall but by weapon.
Of course, some people will read this and think, okay, you should have to buy off the -2 by each weapon, and it should be expensive. Which is fair enough, but it will be expensive, and apply only in a very narrow set of circumstances (you're holding a Large shield).
Second, how do we price it?
The question here is, do you price it as buying off -2 of penalty? Tough, because the TDS assumes the penalty is always on if you try the technique - Kicking defaults at -2, and your price for it is based on that -2. A Large shield gives you a -2, but others give -0. Shields come in 4 basic sizes (Light, Small, Medium, Large) and give penalties of -0/-0/-0/-2 for an average of -0.5, rounded up to -1.
So worst case, where you assume that -2 is so important that you need to charge as much as possible, Shield Wall Training defaults to (Weapon Skill)-3, it's Average (it's an offensive technique, and merely expands who you can defend), and it costs 3 points to buy off.
Better case, using the average, it's -2, it's still an Average Technique, and costs 2 points to buy off.
Best case, you figure that -0.5 average isn't very important at all (affects only some of the uses of the technique) and it's a base -1, and costs 1 point to buy off.
All of those would be per melee weapon skill. So using a Large Shield without penalties is going to cost a lot. A medium shield costs you 1 point of DB but will save you a lot of points.
You can also choose to break off the skill penalty from the blocking for others. If so, you could push all "Block for others" into one perk, Sacrificial Block (from DF15, mentioned above) and put all of the "buy off the -2" into separate Techniques for all weapon skills. That would be expensive (2 points per skill) but it would make it easier to price out.
Personally, I just don't think you get enough for it. GURPS prices getting rid of the -4 for off-hand use at 5 points for all skills, 1 point for each skill. This costs more for less benefit, done this way. Now Shield Wall Training is valuable. It's a good deal a 1 point. But so are a lot of perks (Grip Mastery, anyone? Naval Training, when you fight on ships all day? Teamwork? Exotic Weapon Training?). The thing about perks is they should be a good deal. They should be pretty damn useful for people within the narrow range of utility they have. They're a good tool for dealing with odd cases and edge cases (like your shield training affected other offensive skills), minor rules exemptions, and other "doesn't quite fit" issues you want to deal with.
They don't all need to be equally valuable, just valuable to the character who acts in the manner covered by the perk.
My thoughts on the pricing-as-a-Technique approach.