To start I am NOT a fan of Steam, however there are a few points about it that I'd like to point out.

1, steam installs only with your consent.
2, Steam can be removed from your system.
3, Steam has an offline option that will stop it from uploading anything (note this dose not stop it's spy-ware capabilities!)
4, A third party (ie. not Microsoft affiliated) firewall with anti-spy-ware can block steam completely.

All these, at least, make it a far better option then SecuRom.
Finally, if your running Vista you already have so much "spy-ware" running on your system one more, whether Steam or SecuRom, will make little difference anyway, whether your worried about the hit on performance or spy-ware it's self.