Why do you think that the a clone of bitcoin can success? The bitcoin only has value when someone use them, so the large the group the of bitcoin owner, the larger its value. It is called network effect. For the cryptocurrency you listed, all of them are the technically the same (except namecoin). If the bitcoin has any fallacy that lead it fails, technically, economically or government attack, none of the other can. So the best bet is to enlarge the user group of Bitcoin so that it can be eaier to survived.
You can also consider the case of facebook. The largest advantage of facebook is its everybody uses it. If there is a facebookClone (it is not that hard) and large amount of people start using it, then it will get the same popularity. But why it does not happen? Likely, they are spending many effort and reluctant to give up what they build up.
The growing value of bitcoin are from the work of the people building business, core developers enhancing the bitcoin client and the decades of other developers integrated it into different platform. It is not likely that they will jump to a (almost) the same CloneCoin and give up their previous work suddenly. If one day Gavin do decide so, I will take a look for the new project.
I think there is a chance that some of Bitcoin's parameters aren't "optimal" in some sense (e.g. lead to slower transactions, more expensive scalability, etc...). I don't necessarily think this is true, but I'm not saying it isn't either.
If 0.0001% of bitcoin users use AlternativeCoin, then it's really worthless and useless to branch into. If 5% of Bitcoin users use an alternative coin, it might be worth it to diversify one's holding from 100% BTC to 95 BTC + 5% AlternativeCoin. I know this is currently not the case, but it might be in the future, especially when/if merged mining will be implemented and widely adopted (I know there are concrete plans for this on NameCoin).
As for the comparison with Facebook, it's not completely fair. If only 10% of my friends and loved ones are using OtherFacebook, as opposed to 100% using Facebook, I lose considerablly if I switch. Every minute I spend on OtherFacebook is in some sense a waste of time. With Bitcoin, since OtherCoin is convertible to Bitcoin in some floating ratio, I could always convert OtherCoin --> Bitcoin --> Goods and services. So I don't need every Bitcoin site to support OtherCoin, just "enough of them", or even just enough people to hold and trade OtherCoins.