Quote from: sparkletwist
This isn't quite as "freeform" or "creative" as it sounds. All you're really doing here is using a practical skill as a knowledge skill when trying to determine how much abstract knowledge you might have about that thing. It's something that could easily be distilled into a more concrete rule-- which, of course, is good, except that the "rules-light" game just got a little heavier.
Compare this to simply saying "It involves Firearms. Roll Firearms" without having to mess with any kind of base stat at all. Which way is lighter?
I get the idea... but there's always going to be a want for some modifiers that apply to more than one skill, aren't there?. FATE does it as you're describing, but FATE also doesn't have much mechanism for having general familiarity in a set of skills, unless you use one of your Aspects on defining it, and even then you have to use fate points to call that proficiency forth. In the end we come back to people wanting some way to spend a little more to boost the results of a handful of skills.
My other thoughts about how this worked in practice would be that the player would probably declare they wanted to use a certain stat as their modifier, and the GM would only make an argument out of it if it didn't make sense. I've played games in more than one system that was like that so I thought it'd be fine enough to use here. It seems Seraphine has had similar experiences but sparkle's didn't go so well, if I'm reading right.