I'm one of those dorks that keeps a full gun tool kit in my truck, so I never really *got* the Multitasker (and even if I didn't, there are still tools on that... tool, that I don't get, want, or need). And I've used the contents of that bag at the range on other people's guns 100% of the time, and my own 0% of the time.
For classes and the like, a spare gun is the A-answer, every time.
When out somewhere that you can't have spare guns, having quality, well-built, and well-maintained firearms is the best defense. Yeah, shit happens, but it happens a lot more to franken-guns, home builds, and DPMS/Oly/etc. than it does to quality guns. And "quality" does not mean "expensive". My favorite things that have happened at classes have been the blank stares when a gun goes down from a guy that either bought an expensive gun confusing that for "quality" or who assembled their own collection of parts because the internet said it was easy.
For the AR, Dean Caputo has used the acronym "MEAL" to describe the most common failures, for Magazines, Extractors, Ammo, and Lube. Guess how many of those are avoided by having quality, well-maintained, firearms? guess how many are addressed by having a spare BCG (or just the bolt) and a bottle of lube?
I like a Leatherman MUT (works better as a general-use multitool than the Multitasker, but has some gun-specific tools that I like, plus IIRC it has a pocket clip you can attach), a spare bolt and a small bottle of lube, both of which can be made to fit in storage bins of various AR stocks and grips.
For the Glock, spare small parts take up precious little space, so I'd consider bringing along all of them if it was that critical.