Updating your sid routinely isn't generally a good idea -- unless you want to spend time fixing your system every now and then. Mozilla's updates, on the other hand, are timely, small (supplied as deltas) and require no attention from me. I also like the fact that there's no middle man here. I get the original product without DFSG cuts, weird openssl patches, incorrectly backported Iceweasel fixes, and whatever else Debian faults I can remember ;) I also don't have to wait for a week or two to get all of this from the repository.
I understand the need for the distributor when the package is distributed upstream in source form only, without dependencies etc, but I really see no need for that in case of a self-contained self-updating Firefox -- the distributor here only adds latencies, complexities and quirks. Or so is my experience.
p.s. And yes, last and the least, I hate this idiotic 'iceweasel' name.