There are a lot of reasons why an exact BTC clone may prove to be valuable.
Some possibilities:
- If transaction space is scarce/expensive on the original Bitcoin blockchain, it may prove economical to have multiple lesser/newer block chains for less important transactions
- Far into the future, if subdivision of satoshis is desired, one option may be to make change with alt coins
- An active blockchain used for real transactions may prove to be better for some testing purposes than Bitcoin testnet
- An alternative chain promotes a more healthy ecosystem/economy: if something goes wrong on the main chain (like the blockchain fork a few weeks ago), wealth may flee into an alternative chain, for short or long term
- An alternative chain promotes mining innovation
- Simply for fun!
Personally, I have yet to see a change that any alt-coins have made that I can unequivocally regard to be an improvement over Bitcoin at this point (although some of the changes may prove to be so, eventually). So to me, if I want an additional currency, the best choice is an identical fork of Bitcoin.
I would like to see a new BTC clone launch at every Bitcoin halving.