A website called Smogon.com pretty much decides the tier system of what is UBER/OU/UU/NU. A general idea to understand whether something is Overused is if it has a mixture of Good Stats (one over the base 100 benchmark) and Ability (like Magic Guard) alongside a diverse movepool (like Infernape). You should check that website out and see what was OU for Generation 5 so you have an idea of an "Overused Pokemon".
Yeah you are right, honestly: it all boils down to patience. If you are willing to grind to get your Pokemon the maximum IV's it can obtain then it matters. That one point on a particular stat can spell victory.
IV's are the least important. Here is a mental category you should have in regards to judging Pokemon...
Type (Macargo is obselete because Fire/Rock provides so many weaknesses)
Movepool/Abilities (Slaking and Archeops could have been OU/Uber Potential if their respective abilities didn't hinder them)
Stats (That's what determines the tier system, ever wondered why Raticate never dished out much damage? check it stats to see why)
Yup, I've been competitive battling since 2007. Yeah, 31 IV's means that your Pokemon is at its maximum genetically potential. (0 IV's means least genetically potential stat). You familiar with the tier system?