I don't think any currency will surpass Bitcoin, because it has the early movers advantage. Why would anyone use LTC when BTC is more commonly accepted?
It is often times more profitable for a miner to mine different coins and then exchange them into BTC, esp. for those that mine with a GPU opposed to an ASIC. Also, for some, exchanging currencies can be a very profitable endeavor. There is also the possibility that another coin might become accepted along side BTC, or even overtake it (although unlikely). Just a few reasons some people like to mess with alt-coins, such as LTC.
I also think that other currencies will not surpass BTC in value, but there is always the possibility. That does not make the other alt-coins have useless or of no value, obviously, so there clearly are other reasons one might have an interest in coins other than BTC.
