Well for some it is a perocative thing, I ahev heard that some use them all, some just use Spybot search & destroy along with Adaware, but most people prefer to use them all.
For one that finds spyware, one will find what the other won't, that is why it's good to cover your bases with using the top 4.
spybot search & destroy/adaware/spyware blaster & CW-shredder.