Aside from the obvious heavy load on the server, is it bad to have several concurrent AJAX polls going on at the same time? Or is there even a limit as to what a browser/server can handle?
Example (all AJAX polling)..
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Function A is on a 3 second interval pulling for new message for a global chat system (like Facebook/MySpace).
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Function B is on a 2 second interval for pulling updates and what actions to show the user (such as in an online poker application).
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Function C is on a 4 second interal, but also pulling for new message for a separate messaging system.
Browsers used to have a limit of 4 (or in really old cases 2) concurrent connections per domain. Most modern browsers have upped this to 6 or 8 (based on my own testing and some supporting reading).
So long-polling connections can eat up those slots in the browser and potentially prevent other things from downloading as quickly.
Otherwise, it could be a bandwidth hog for people on slow connections (dial-up or cellular networks).
There may be other drawbacks, but those are two big ones I see.