The reason BCVM22 noted my statement is because it's the most common argument, and one that has no real fault.
As for multiplayer, I generally split hackers into three groups; legit hackers (a bit of an oxymoron yes, but still), n00b hackers, and data hackers.
n00b hackers are the ones who hack 999 on all stats, shiny, illegal moves, etc. Note they don't have to do all of this to classify as one, and that being a n00b hacker isn't specifically bad... unless you do it in multiplayer against someone who doesn't know you are hacking.
Legit hackers follow the strict guidelines the games provide when making their pokemon. Due to this, they are indistiguishable from normal players unless they outright say it or they stretch the limits (31 IVs on everything for example). They also are the people who hack normal things like all TMs, max money, ect.
Data hackers on the other hand are the people who hack into the game to discover secrets, such as EVs, IVs, and other things like that. Nobody has a problem with these guys. Some people don't even consider them hackers.
Since data hackers are usually left out of conversations like these, and since legit hackers are impossible to find normally, most people associate hackers with n00b hackers. And that's where a lot of the problems hackers undergo come from.
Anyway, as a final point, I'd like to address the ridiculous statement made by many people: that legit hacking for EV and IV trained pokemon isn't right because the player didn't work for it. Excluding the fact that since the only reason you're training your pokemon like such is for battling, and the fact that people arguing this are essentially quoting the rival from R/G/B/Y ("Gramps! Why does he get his pokemon first?"), the only difference between legit hacks and normal pokemon is the time spent on it. And really, if you're angry at people not having to spend hours on training a team of 6 pokemon, you should be more angry at Smogon.
/rant.