I’m talking precisely about rules that are fuzzy with regards to the setting, not rules that are hard to understand or anything.
An example of exact rule: Climb in d20. It tells exactly what one can achieve with a climb roll an how difficult it is.
An example of fuzzy rule: Profession, not considering the part on earning money, in d20. It tells roughly what character can do but does not include particular difficulties or durations or such (because there would be too much to list, but that is not relevant).
My argument is that exact rules are prone to breaking the game and hence being ignored. I probably could also provide some arguments that tell exact rules are actually preferable, I don’t actually think they are nearly as compelling and so would likely set up strawmen anyway.
The breaking happens when some task clearly in the province of the skill but not covered by the exact rules comes up. Like, say, two characters are trying to climb atop a 50 feet castle wall. The important bit is who gets there first. You could habe both chars roll those 7 to 4 climb rolls that the rules imply. Or you could simply roll one opposed roll. Seven to four isn’t lot. What about if the thing climbed is 500 feet high. Still rolling? I’m certainly not.
Likewise: Mass combat in any game without specific rules to handle it. Creative use of skills, spells and other character abilities. Diplomacy.
With fuzzy skills, I can simply ask for a roll and state difficulty, together with consequences of failure and success, with little need to consult a book or such. Like using profession (general) in a war situation: Roll it as a special aid another that affects the entire army if you beat the opposing general. Or roll to guess the ambush, DC 20 for it being possible but unlikely for a random person to notice. Or perform DC 15 to get the lady talking to and interested in you. Or roll farming to get your crops look impressive enough that the samurai believe you are able to pay your debts, so you won’t be slain right away.
This does mean that there won’t be a one-to-one mapping between the char’s running speed and relevant abilities. Or the estimated jumping distance and relevant numbers. Instead the GM makes a call about the difficulty of the feat (jumping that is pretty easy, DC 5) or asks players to make the call (“How difficult is it to jump 5 ft. without a running start?”). Or doesn’t even use clear measurements, because it is not like the characters know those anyway. So, pit traps can be measured straight in difficulties (DC 15 to jump over, 25 to jump out of there once in, +2 if someone is ready to grab you if you get high enough, +n if smart tools are used, …).
I certainly prefer fuzzy rules. They don’t create logical inconsistencies or huge rollfests as often as exact ones. They do give more responsibility to the GM and possibly players, though, which is something I can and will live with. YMMV. Credit goes to Thalin for making me think.