I figure you can do whatever you want so long as you only screwup your game. when it comes to things that involve other people's games:
gts: I don't think it's okay to trade a hacked pokemon unless the other person knows it's hacked.
Battle (with other people obviously): I think it's fine to use a hacked pokemon so lang as it is possible to be obtained without a cheating device.
and shiny: some people don't have time (or luck in my case, I've played over 700hrs on my emerald and still not seen one) to search, and what good is a shiny poke really? It has the same stats, moveset, ability, etc. so yes hacking to get a shiny is fine in my opinion.
The first sentence says it all though, why should other people care how bad we screw up our game if it's not hurting them or their game? So long as AR people don't put the other people at a disadvantage, I think it's fine. some people say "u is not enjoyin teh game as much.", but I say that people like myself who use ARs are just enjoying the game differently. To have fun with a game don't you have to enjoy it? I say yes, so I don't like to be dragged through kicking and screaming just to have someone say I enjoyed it more. I rest my case.