The crumbs still build up underneath the keys though, along with oily skin flakes, dandruff, the odd eyelash and bit of stubble... maybe a few nose hairs/mucus, earwax and tooth plaque too if you've got some particularly bad habits. Us humans have a fairly large amount of manky bits fall off us every day.
Eventually all this inevitably congeals into layers of tacky gloop beneath the keys, making them impossible to press, finally requiring the user to go through the tedious process of removing each individual key for a good cleanout. Or chucking the thing and picking up another cheapo one for £3.50. That's the option I take once every 6 months or so.
A bit of light bacteria is good for the immune system, the only problem is when users share keyboards. This anti-micro-gribbly tech would be much better suited to terminals and businesses with multiple users per PC.
Anyway, I cite xkcd: http://xkcd.com/237/