"antivirus program will try to block data-stealing spyware, and most virus fighters block — or at least warn you about — adware and other unwanted programs"
Although this is true that spyware will be blocked by certain other security programmes,it will still build up on your pc clogging up the system and using resources.
The way different people use a pc often dictates how much spyware they are exposed to,so doing away with anti-spyware software would not be a good idea in 90% of cases.
I would guess that GANDALF <|:-)> only uses his pc for light surfing well known sites(fell free to correct me if wrong)but many users download and google for various free and illegal programmes e.t.c.which can sometimes lead to malware infested sites.
Also it is quite surprising how many legitimate sites have spyware put on their sites.
A work collegue scanned his pc with anti spyware pro on my recommendation as he had no anti spyware software installed.It came back with 194 infections.
Do without it?..not me,i'm a cowardly,cowardly custard.