Well, first, the fact is that it's up to the referee to run the mechanics of the thing. Using the car example again, I could ask you whether you've ever done a one eighty skid turn in a tractor trailer; you could answer no, but you have driven a tractor trailer; you could answer no, but you have done a one eighty skid turn in a flatbed. Maybe you've done both. At that point I'm going to tell you to roll the dice. I don't have to tell you whether you are 1) applying your skill at driving tractor trailers to trying to do a skid turn at a heavy penalty; 2) applying your skill at skid turns to doing one on a tractor trailer at a heavy penalty; 3) teaching yourself the skill of doing skid turns on a tractor trailer, which is probably a heavily penalized skill itself. What matters to you is that I treat you fairly; what matters to me is that I treat you fairly, and that I facilitate future play.
Now, if I think that the future of the game (something I will know better than you, although neither of us will know with certainty) is going to have you doing a lot of stunt driving in tractor-trailers, it's going to be much easier for me in the long term to give you a skill in tractor-trailer skid turns, so that as you get good at that we can use it in play specifically. In that case, I oblige myself to be rather lenient on my definition of "new use", allowing a bonus any time you use the skill on, perhaps, a new surface, or with a different kind of trailer or significantly different load. I might instead think that the odds of you having much use for such a skill that narrowly defined is minimal, and I'm better off letting you use your skid turn skill and giving you new use for the tractor-trailer--in which case I have to be stricter in my assessment of what constitutes a "new use", saying that yes, this is a new use for doing a skid turn with a tractor trailer, but you'll have to do it with something like a cement truck or a dump truck to get another new use credit, because we're talking "skid turns" not "tractor trailer skid turns". I might think that you're not going to do much driving at all in the future, and it's foolish to clutter your paper with a lot of individual skills, so instead I'll just let you roll this as a driving skill use, note your paper that you have accomplished a skid turn in a tractor trailer, give you new use for driving for that--but again, I'm going to be stricter in defining "new use" after that, and skid turns in heavy vehicles won't count again.
Note in the particular case of driving skills, a referee might take different approaches with different players in the same game. I've learned enough about you already to know that driving is a priority for you; I'm probably going to put a lot of attention into the details of your driving skills, one way or another, so that when you are, somewhere down the road, barreling down 95 in a gasoline tanker in the middle of the night you can suddenly decide to do a one hundred eighty degree skid turn, and that will be a skill on your sheet for you to roll. Scott won't have that skill, probably--he's never shown any particular interest in having or operating a motor vehicle, and I'm unlikely to put him in worlds where that kind of detail will matter. Thus it facilitates play for all of us that I get to make those decisions individually.
Of course, it could be hard-coded in the rules that all driving is handled the same way. That would mean that the game worked better for players (and referees) whose games went that way, and worse for those who don't. More fundamentally, though, stop and think of how many hundred of thousands of possible skills there are. How many specific skills could be discriminated under the heading "gene splicing"? How many under "design computer"? How many under "operate explosive-driven slug thrower"? Already lists and descriptions of categories of skills comprises over a third of the book; we'd need an encyclopedic listing to cover the detail of every skill. Multiverser trusts its referees to make these judgments, because the referee knows what works for him. If a particular referee wants to class every kind of gunpowder-driven gun from a matchlock to an AK-7 to the guns on the deck of the USS New Jersey as the same skill, that suggests that for purposes of his game differences between guns are not going to be that important. If he wants to require you to have separate skill in the Colt versus the Eagle, that suggests that this is a very significant skill in his game. I'll give him guidance on this, but I won't dictate it; it's more than I can do.
Why doesn't the player's opinion matter? That's the sort of question which in law would raise the objection "assumes facts not in evidence". What makes you think that your opinion does not matter to my decision? I just showed above that the fact that driving matters to you means I'm probably going to discriminate your driving skills more finely than those of others in this game. Your "opinion", expressed through your choices in play, will influence my handling of your situation. In fact, if you were to say, as you're barreling down 95 in the middle of the night, "I'm going to teach myself to do a one hundred eighty degree skid turn in this tractor trailer," that's going to tell me that in your mind this seems like a new skill--whereas if you said, "Having done a one hundred eighty degree skid turn in a flatbed once, I'm going to attempt to do the same thing with this baby," that tells me that you think it ought to be a new application of a known skill. You've thus given me your opinion on the matter, and I take that into account as I decide whether it's a new skill or a new use of an old skill, or just the use of a known skill.
Someone has to make the decision, and has to do it quickly to keep the game running smoothly. Sure, I could stop and ask your opinion, but that only slows down the game, and if we disagree, it slows it down more while we decide how to decide. We know how to decide: the referee decides. That's his job, to decide how the rules apply--and that's why in Multiverser we call him the referee.
--M. J. Young