Let me have a crack at this :D
On Wi-Fi I think that it is worth it. Mainly because it allows you to outdo many of your opponents that do not use EVs, using your increased stats to decimate most Wi-fi battlers out there. however you can still be moderately successful without using them, so t's up to you to choose that one for yourself.
No, it is not worth it. To be successful ingame, all you need is to not share to many weaknesses and have a few different types to counter whatever you need to. Otherwise, there is no reason too as to beat enemy's you just need to grind those levels more. EV training isn't that important, and most likely you will pretty much be adding EVs onto Pokemon anyway as you fight constantly different Pokemon.
Hope I helped :D