Disclaimer -- this is totally unscientific. Just one site, just one instance, with just 3 keywords.
A particular site I manage does well in the SE's, and has been #1 on MSN for 3 important keywords.
Full knowing the major SE's will index a site even without a
<META NAME="ROBOTS" CONTENT="ALL">
But hey, who knows if there's some search engine out there that it may help in, and a few extra visitors every now and then certainly won't hurt. After all, just because the majors don't use them (unless you tell them "noindex,nofollow", which even then is debatable sometimes), they certainly wouldn't knock you for using the "index,follow"...or would they?
<META NAME="ROBOTS" CONTENT="INDEX,FOLLOW">
So I go ahead and throw it in, just to see if it makes any difference in traffic/referrals.
In a couple days the 3 keywords getting first place in MSN suddenly go to page 2, in the 11-20th range. Hmm. So I remove the tags, and in a week they're back to number one. It's the only SE where I noticed a difference.
Just a coincidence? Probably, but the timing sure looked fishy, since no other changes were made, as far as code, content, or IBL's (that I was aware of). Would be interesting to run a test with a number of different sites.