Mob Spawner/Mob Grinder Rooms are very much so frowned upon and will not work, at all.
To sidestep the general headachey/laggy nature of mob grinders, on Summa Crossroads, Mob Kills will reward increased loot, gold, and (experience?) (we're on a hard mode server, hunt your prey--don't cheese)
On top of that, Mob Spawners will reward 30 levels worth of EXP (From Level 1) as well as a Diamond Block.
edit: no confirmation on increased experience, it just seems like it in comparison to Vanilla Hardmode.