[moosehawk]

There are mining pools available that automatically switch all miners to the highest profitability coin. People who utilize these pools will typically immediately cash out to less volatile options, either to BTC or fiat. (Multipool.us and Middlecoin.com are just 2 examples)

Some miners aren't in it for the immediate profitability though; While you may be able to mine .025 BTC worth of AltCoin A per day pretty regularly, I may choose to mine the much less profitable AltCoin B that only nets me .01 BTC each day because I believe holding it will make it much more valuable N months from now.