Back in my first GURPS game, wizards wore robes and carried staves. I didn't tell them to do so, although I didn't tell them not to, either. They just did. Daggers, robes, staves - they'd shun armor and wouldn't think of using a non-staff weapon. The availability and utility of the Staff spell helped, but it was more than that.
All of my players at the time were AD&D vets, often from my own games, and my first couple of games were set in first Greyhawk and then in the (then just released) Forgotten Realms.
So even the NPCs were equipped that way. The only guys who would skip the staves and use swords and bows and armor were the couple of "fighter-mages" people made up, usually elves (they had Magery in their racial package). They tended to be fighters first with a couple of spells at useful levels, not really wizards.
It was only my next group that realized what "no classes" meant in a full sense. We ended up in very short order with wizards with leather armor (mail was too heavy, and would slow down a weak wizard too much), one-handed staves (usually a 3-4' jo), and shields. Once the first guy took a shield, I'm not sure I've seen more than one wizard without one since. Even that one is a "not sure" because I think he started with one but abandoned it once he felt like he needed a free arm for Iron Arm or something.
All of our DF wizards use them, too, and not just because it gives them a place to put their shield lectern for mapping.
Why use a shield?
Basically, it gives you Block. Block is useful, just like Dodge, versus missile fire. The DB of the shield adds to Dodge, too, and the parry from Smallsword, which you're using to wield your short staff. It also gives you a second weapon if you need it, for bashing or slicing, although it's not a moneymaker attack for your typical weak wizard.
You give up some stuff from the quarterstaff, though. Shorter reach, and thus a slightly worse effective range on your spells with a Staff-enchanted one. Staves have a sweet +2 to Parry, and get favorable multiple parries in GURPS Martial Arts. They also let you reach out and touch someone with a Melee spell from behind an ally. All pretty good stuff.
But, conversely, your Parry probably sucks anyway even with it. And if you're in melee you're probably going to die. Parry is the last resort after blocking spells and flat out staying out of melee, at least in my games.
Not only that, but the usual DF counter to wizards is "just shoot them with meteoric iron arrows." Meteoric weaponry is unaffected by magic, so it will ignore the annoying Missile Shield and dangerous Reverse Missiles. While there are blocking spells that will surely deal with one as easily as any other attack (Blink or Phase, for example) it won't help against multiple arrows. For that you need a good solid Block or Dodge to fall back on - and the DB bonus of your shield adds to both; not only that but Prediction Shots can't lower Block, only Dodge. And even if you rule that any Deflect spell doesn't add DB against meteoric arrows, you've still got the native DB of the shield. Even if it ignores it, a +1 to your shield's DB is a mere $2K away in DF.
So all of my players' wizards like to grab a shield, perhaps grab Shield Wall Training, and cover behind it casting spells. Because in GURPS Dungeon Fantasy, wizards don't have Shields: No and aren't restricted to Staff, Dagger, or Darts.
Melee Academy: What's the Tradeoff for NOT Using a Shield?