This is a
really tough question to answer because it incorporates two factors.
1.) Perceived value
2.) Actual value
Our definitions of both can be wildly different. Let me just tackle a few skills I saw mentioned in this and the other thread.
The measure of the difference between these two values I'll give in 'cookies of disjunction'. A very scientific methodology!
Treat Injury
Some guys voted for this. I don't understand it. Its perceived value is, for me, exactly the same as its actual value. DC 15 for 1d4 hp once per day. It seems like you know exactly how useful this skill well be when you take it. Sorry guys, I'm gonna have to disagree if you voted this way. 0 cookies.
Survival
This is a crap skill if you are not playing in a natural environment. But don't you know it going into the campaign? Or do you take it often believing you are going to be in a non-urban setting before the DM pulls the tricky-switchup on you.
I take survival only for mutant characters that have the 'Scent' ability. Survival effectively becomes like a third awareness check along with Spot and Listen - it represents your highly tuned sense of smell. I know more or less what I'm getting out of this bad boy.
Because in some cases this might throw you (or your game might start in wilderness and end up urban) a single cookie of disjunction is appropriate
Disguise
I'm really surprised this is coming up. I get as much use out of this skill as I'd expect. Characters that I give high disguise to (charismatic characters) will basically only have a chance to use it when the other players aren't about because the other players will have comparatively poor disguise. If you are being recognized there is a likelihood the other players will to. As such it is a difficult skill to use in a group game (unless for some unfathomable reason everyone has taken disguise).
Yeah its not that useful - but how often do you disguise yourself in real life? 0 cookies.
Balance
I think I agree with Masada. I do not, as a GM, have a clear use for balance and if a PC of mine uses it either:
1) there is a rope bridge or the floor gives away sharply or the starship shakes - everyone has to take a balance check. You can't be too lethal about it because most players don't have balance as a skill.
2) the character has a skateboard or other balance dependant equipment. He has high balance to do special moves.
90% is case 1 - when I use a balance check it is a fluke occurance. A trap or something. And because of this I have to weigh in the factor that the character with balance won't be taking it.
Compare it to say JUMP. With the Jump skill a character with high Jump can Jump when he chooses to. But in the normal run of things a character with high balance will not necesarily ever use the balance skill without GM intervention.
This gets 3 cookies of disjunction
Investigate
I really like the idea of a detective character. I often take Investigate. I've actually only now realized that GMs can't tell the difference between Search and Investigate. Investigate is for profiling and crime scene analysis - because GMs are unprepared for fingerprint IDs and stuff unless you are playing a very police oriented game Investigate is basically useless even though in real life it is the primary way of catching bad guys.
I give Investigate
five cookies of disjuntion
Knowledge Skills
Knowledge skills are great in premade adventures when the GM gives the players more background info on a group or organization based on some preassigned knowledge skill. For instance when playing a gang game that was premade Knowledge (streetwise) can be very useful as the GM may have some tables that allow you to roll a 1d20+ streetwise and you may know some stuff about local gangs just because you know.
Unfortunately most GMs are not that organized. They have no tables ready and when you roll they have to either say "That's not appropriate now" (which effectively makes your skill useless) or they have to do some kind of "ad-hoc info" which will never be quite what you want because it will ruin the plot information distribution times for the GM.
In conclusion Knowledge skills tend to have a high disjunction between what I think they should do and what they actually do. But with a caveat! Know your GM. An organized GM will make good use of the Knowledge skills.
Two cookies of disjunction