But Coinbase and BTC-e are not pools, they are exchanges/onlinewallets. That's awholenother beast.
A pool can only steal as much as it takes a miner to realize there's something wrong. So if a miner checks the statistics even just once a day, the damage is very limited in size. And the damage of reputation a pool might have from this is much higher than the actual profit that stealing might bring.
(Sorry, should have been 2-of-3 multisig, corrected it)
And the pool cannot steal anything if you're in a 2-of-3 escrow with say, the mediator being a trusted third party (reputable person on this forum, whatever).
So... yeah. I don't think that currently the software is 100% "pool impossible" or anything, it's more "bonded pool mining amenable, with increased incentives for solo mining".
The bigger issue will come when someone decides to multipool SpreadCoin, I'd guess... so you'd mine for a bit on their server for free to generate your initial bond (mining whatever currency is currently profitable), then that bond enables you to mine SpreadCoins but the payout is in BTC. Miners tend to be bottom feeders and will mine whatever is consistently bringing in cash, if the automated multipool is set up correctly I believe you can amortize the risk enough to remain profitable. But for the moment this is a lot of effort.