If EXP for MCMMO was allowed to be gained from spawners in the normal world, it would encourage illegal mob traps/grinders. (Not for MCMMO - but just in general.) Blaze probably wouldn't, since no trap there would be easy to make.. Let alone last very long.
Shouldn't. Go spend a half an hour leveling up swords in there at level 5, good luck leveling up, lol.