The pools contribute to the problem but currently the largest volume of transactions comes from the faucet.
And also from people who are abusing it, and are funneling cash back into their primary account, often without TX fee...
http://blockexplorer.com/a/8i3b7nXppE(View transaction detail -- many 0.05 receipts.)
This came out just now in #111264 when someone 'unclogged the drain':
http://blockexplorer.com/t/APn3cXG9Z1# Number of inputs?: 181 (Jump to inputs)
# Total BTC in?: 9.05
=> 181*0.05# Number of outputs: 2 (Jump to outputs)
# Total BTC out?: 8.72
# Size?: 32.661 kilobytes
IMHO though, if anyone could maliciously generate a comparable volume of free transactions, that's still a potential issue, even if the faucet is fixed.