but I do run SpyBot Search & Destroy, SpywareBlaster and Microsoft Antispyware on the same machine and have not experienced any problems.

But in my case my Spybot S&D does not run in the background, I turned off its TeaTimer background checking, and just update it and scan with it regularly. And SpywareBlaster works differently anyway. It immunizes against spyware deep inside the registry without needing to scan.

If you don't experience any problems or conflicts then it is a good idea to have more than one anti-spyware.