The mob arenas--from what I saw on the test map--are an excellent source of exp. On the test server, a partied user could hit 55 in 30 minutes tops. I don't know if this would stop abusers or not, but it's definitely going to improve leveling experiences on Herocraft.
What we'll likely see with 2-3 arenas, is that they're always full, and always by the same group. This'll end up being a problem on it's own when the time comes, but I'm really hoping for a lot more arenas to choose from, more variety too. (Think platform over a death fall with skeleton spawn points on ledges, slime spawns on the platform.)
I kind of doubt offering an alternative will stop spawner abuse. Some people will still do it, simply because it's easier. Maybe we could flag all mobs that are spawned at a spawner, and if they take damage but can't hit someone within 5-10 seconds, they explode with the force of a dozen creepers? /fantasy