There are a couple of interesting ideas come up in this thread that I have never seen before:
1) the words listed as keywords influence the adsense ads shown
2) misspelt words should be put in as keywords
Is there any evidence or supporting links for either of these? The first suggests that I could 'encourage' better ads to be shown, and the second would be very useful.
I never bother using the keywords tag (just me being lazy) but if both these statements are true they are a good reason to add them back in again.