I can't see a good reason to do so.
I give my Yahoo email to my seniors when they graduate, and a few actually make use of it. One or two students have figured out my email from my blog profile page--no big deal. At most they've emailed me to let me know they found it. Any other student who has my email uses it extremely rarely, and it stays professional--job references, college or military information, and the like. I remember looking my teachers up in the phone book; I wouldn't have dreamed of calling them, but it was funny, somehow, knowing they were listed.
With internet chat, though, lines can get blurred. Is there really a reason to chat with current students online? Full disclosure: I just finished a Yahoo Messenger chat with a former student of mine who attends one of our service academies (and talked on the phone to another such former student last night)--but I'm sure we can all agree that that's not even in the same league as sex chat with a current student.
I'd ask what such people are thinking, but it's clear they're not.