They aren't tags, they're
attributes. IE is the only place where they pop up as tooltips. (Other browsers behave the way they're supposed to and don't pop up the alt attribute -- IE got it wrong.)
But users of IE aren't the only ones who will see them...
Visually impaired visitors will have them read out loud to them, and anyone using a browser with graphics turned off (cell phone/PDA, maybe?) will see the contents of those attributes written out, which makes them a
terrible place to stuff with misspellings and alternate versions of your keyphrases.
My

--Torka