No one can steal your funds, if they don't have your private key.
Just because you are good man.
Instead, if you are a bitcoinspinner user and I have hacked the server / killed the developer or I'm Crazy-Jan, then I can make a fake account on some markets and offering you ( and many other users ) bitcoin for money.
I'll make 3/4/5 fake blocks and I'll sell to you many fake bicoin, for your REAL money. ( and then running away ... )
Can you understand it now?
Connecting all/many clients to only one server ( or more controlled only by one person ) has the same problems as giving all mining power to only one pool.
Anyway, I read that slush has took the bitcoinspinner code and he is converting it to works with the Electrum server, I think this is the best choice.