A lot of the magazines have recommended using 2 antispyware programs because no antispyware program finds everything. What they said was pick any two and use them both and hopefully the second one will get anything the first one missed.
Whatever ills Defender may or may not have it is a full time/real time service to block spyware. Whilst I agree that Spybot and AdAware are also useful the free ones are only scanners for intermittent use. Incidentally I have had no problems with MS Update in Defender.