Because right now slush and his server are each a single point of failure. If either one becomes unavailable we have to go back to mining individually. OTOH if the source is available any one of us can take over in that event.
yep, that's true, just like MtGox and MyBitcoin, all single points of failure and still not open, or "mirrored".Quote
I expect that over time we will be able to tell whether or not a pool operator is cheating.
and i expect that over time slush will release the code anyway, cuz he got bored of people asking (or coding himself), 
or other pools coming up using different code and other methods, maybe puddinpop improves his code, who knows?
of course it's nice to see for yourself, from the code, how it works
and it sure would be nice to have multiple developers working to improve it, some miner-devs to implement new features,
but it's still his choice.
anyway, i just take what i get.