CP vs IV: I've heard this a few times. I'll try to be complete.
My first thought is wtf?! IV always duhz!!
Cause...think of it this way. CP is how good a Pokemon is RIGHT NOW, and can be bought. IV cannot. Through solid gameplay work CP can be taken straight to the top. But iv is more plainly how good a guy WILL be.
So, my IV better always argument does just fine for guys that you want to invest dust in. However, that is only half the gym metagame. Esteem grinding into a friendly gym is a whole separate ballgame. Something where you don't have single gems of powered up type-hate, but squads of dudes where you appraise, get the top message, fast tap the rest, evolve, roll dope moves, and keep forever.
Even though you never intend to spend a shred of dust on it, it sits at such a stock cp range that you keep using it to troll the hell outta the same rhydon, Gyarados, Snorlax, Dragonite crap that you continually run into.
I trim the hell outta certain stuff sometimes but I just cannot bring myself to get rid of a pump vapo, thunder jolty, throw/edge golem, beamer parasect/wigglytuff/Furret/raticate...you get the idea.
My point is, for those guys, cp matters. You get a pump vapo around 1500 cp-ish? Great! I'd say about 20 of those before you might trim, lol.
Note: I still end up replacing with good iv stuff that, well, lucky, lucky your also huge. Just...nothing wrong with evolving and holding a huge snubull, or something like that.