A lot of the motivation for the 1MB limit was a fear that if it weren't limited we'd have people building gigantic blocks just to clog up the system. If someone with major hashing power started building giant blocks every time he got a block, he could DoS the system just by making the block chain too large for most people to store.
How exactly would this be done? Where are these transactions coming from? Could this "bloating" attack be detected and these blocks rejected?
It would be better to prevent this some other way and remove the limit entirely.