There is one reason not to use them - they don't work with IE6 transparent PNG scripts - or at least I'm yet to find a trans PNG script that does work with sprites.
Either you go for an opaque background, or with the transparency fix applied the entire image is loaded (rather than just the segment of it containing the relevant icon).
If you are using, say, white background sprites on a page with a white background, this is no longer an issue, however.
1:43 pm on Sep 28, 2010 (gmt 0)
Good point milosevic. If you don't need transparent PNG background images, then this won't be an issue for you. Alternatively, you could serve up different images for IE6. Fortunately, the days of needing to support IE6 are finally nearing an end. :)