The problems McAfee has with Spybot are probably that they think free anti-spyware software is a competition they can't beat in price, so they'll just beat them in inproper competition by listing it as incompatible and forcing users to uninstall it.
Or in short: "political" (financial greed) reasons by McAfee.
As with every software out there, using more than one on-acess scanner can cause problems, which would mean that if something does not work, you might want to try to disable TeaTimer. But there's absolutely no reason we are aware of that would affirm a need to uninstall Spybot-S&D.