You don't need a whole bunch to reset EVs after one fight. I think I managed to reset Yveltal with 3. Most I can imagine you would need to reset after fighting one 6 poke trainer is 6, unless you managed to get pokerus on it or put EV stuff on.
I'm not talking completely reseting a fully trained poke, that takes a lot of time in the berry field, I'm talking resetting one that was just forced into a fight or 6.
Only issue with that: When you have one of each berry that resets, and only get 2 per plant...and they take like 3 days to grow...and you water them regularly...it gets annoying.
Thankfully I'm so carfeul with raisin in EVs that I rarely have to reset. Those reset bags are invaluable though because my imports were EVed before I knew how to do it right...so...yeah reset bags for the awesome second chance at win. to bad they don't like to apper for me...ugh...
I only care about EVs myself because in the (extremely unlikely) event I do find myself using my team online, like say to help a fellow look at their team's stats for purpose of finding out IVs the judge doesn't comment on, like I did last time, then at least I'll stand a bit of a chance.
Also it still gains EXP, which could diminish trade value to some folk as they prefer purely untouched Pokemon. Yes, they are out there, sadly.
Oh, I was totally thinking about Black forcing me to use Reshiram. This is the confusion you cause when you release two gens on the same system, Nintendo.
You know that let-it-faint tactic of mine? I invented it that very moment my Vulcangel (Reshiram) didn't go to the box, let the thing faint, and used my level 80+ sweepers on the other legend to ensure it didnt so much as get a lick of exp or EVs. Toss pokeballs if you have to, but waste your turns. I also did it with my Xerneas.
I am going to be playing Black again anyways, I wanna replay it, and now that i have finally imported my salvageable Pokemon, I'm good to go.