You don't need antimalware on Windows 8?

You definitely do need a 3rd party antimalware solution on Windows 8. The whole point is to push the probability of infection further along the tail end of the curve. Good computing habits won't help you in such cases. It doesn't matter if 99.9% of careful users relying on Windows Defender report having no trouble with malware for X amount of time (with X being large here) - the point is to ensure that you don't fall victim to edge cases, since only a single successful attack can cost you dearly.

Also, those who write malware for Windows 8 will certainly interact with Windows Defender and the smart coders will make sure that their 'product' can defeat it, if only imperfectly at first. It's much harder for them to target several different antimalware solutions.

These are obviously 'at a distance' considerations made without going into any Win8 specifics, but as a long time Windows 8 user (if there is such a thing), I can say that Windows Defender has unexpectedly stopped working or failed to update on several occasions. This doesn't exactly inspire confidence, so I chose to rely on a 3rd party solution. The bottom line is to use your own judgement and (as always) to at least consider sensible contrarian options.

(Note: I'm not a specialist in this, so I can't say how effective Windows Defender is in combating malware (not least because AFAIK you can't run Windows Defender alongside some 3rd party solution.))