We used to be able to mine change and place spawners which players would use for mobtraps. I believe this was removed because -1- it makes the game to easy and -2- a couple of players making 60+ spawner grinders which put lots of strain on the server.
I think the other problem is that any non-VIP member cannot place a MOB spawner ... hence the explosion.
I think it was a protection built in to the server since VIP's were stopped from mining spawners, to stop them selling the spawners onto "normal" members. Thus even if you got one as an xmas pressie, your best bet would have been to sell it on (though I'm not even sure VIP+ can place a spawner now without an explosion).
vips cant place em, chista had a bunch and said they all blew up
When the ability to mine and change mobspawners was brought in was when the update with cave spiders was brought out. So originally what was only intended for maybe a few spawners there was suddenly vast amounts of spawners available (due to all those lovely lovely spider spawners).
But as has already been said some people did have grinders with 60 mobspawners in them and it just caused so much lag, hence why they can only be changed and not placed (just to out of those who have a stockpile of mobspawners (like i did) from being able to build grinders still).
Its a shame the cave spider spawners were so common, as we may have been able to still mine them, but alas we can't and tbh id rather not have huge grinders again.