We don't bother with alts at all (US 19th 25man) but that's more to do with the fact that we only raid 12 hours. Not really enough time for multiple week 1 clears. If we raided 20 hours I think we would, sure.
We have brought alts in once or twice when setup demanded it (we lacked immunities for Lei Shen so a warrior swapped to rogue for cloak and symb cloak). They're nice to have but are a pretty big timesink.