My own question: The bitcoin network has operated well under the limit so far. If arbitrarily increasing the block size were so advantageous to certain miners, why hasn't it already been done? We should have seen the limit maxed a long time ago, yes?
Increasing it beyond 1 million bytes would require a hard fork. We are close to reaching the 250k bytes "soft limit" of bitcoin-qt however, mostly "thanks" to Satoshi's dice.
Writing custom rulesets requires work and this was not necessary so far, there are actually no real specialized "mining" bitcoind implementations out there as far as I'm aware.
Everyone running a full node has to store currently ~2.5 GB of SD transactions alone.
Currently the rules in bitcoin-qt are quite tight concerning "spammyness", also quite a few pool operators are as far as I see it rather interested in the long run than risking something by making a quick change for a few extra coins (=accepting more spammy transactions with and maybe even without fees).