mob spawners of all types exists already in the game, They are just not given to us in creative to place. I dunno why its this way, and its the same way on the PC version even at its current version, however mods like TMI/and NEI will give access to mob spawners.
I believe that when we get trading with villagers one of the trades they will offer is a 'pig spawner'. Spawners no matter how many you have still must abide by the mob cap limit.
The only way I have found to break the mob cap limit is with silverfish and its no easy task as one needs to build a machine that allows silverfish to infect cobble or stone blocks and store them away in a huge block, with that done; you begin breaking the blocks and damaging the silverfish which causes a chain reaction of other silverfish to break out and before you know it you have ALOT of angry silverfish. This is actually the most exp i've seen in one chain reaction, enough to go from 0 to nearly 57.
No, villagers do not trade any spawners of any kind, unless you hack villagers like in many maps, such as the Ant Farm, where you can get enchanted OP stuff from these villagers. Would be awesome if you could use spawners though, like how BloodyPheonix wants pig spawners that can be changed with spawn eggs.
While the mob cap would keep the lag in check the animations the mob spawners have could be laggy. There is the fire and the little mob inside spins around. So imagine like 50 of them in close proximity to one another. They would need to create a limit on total mob spawners, like how they limit active TNT.