I'll tell you why (I read it on a forum).
NOD32's scanner doesn't scan all areas of the hard drive. It scans the main areas. This isn't that good, from one perspective, but NOD's heuristics and virus-catching capability are unbeaten. So, if it doesn't catch a virus hiding in a directory, it will inevitably catch it when the virus decides to spring up.
I had researched this alot, and this was the common notion (and the notion was also supported by a NOD32 forum admin).
I like Kaspersky, but
1) it's keys keep getting blacklisted, and
2) it leeches quite a bit of ram
3) it's scanner isn't as fast as NOD's (though it scans all areas of the harddisk).
Here's what I would suggest: leave NOD in your PC, but run an occassional spyware cleaner - that way, the spyware cleaner will check every nook and corner of your hard disk, and NOD will simultaneously scan all files that are scanned by the spyware cleaner.