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.