The only way an alt-currency survives is if it has some new feature that is impossible to add to Bitcoin. That only happens in very rare instances, or in situations where Bitcoin is so large and stable that it isn't worth breaking. There are also some situations possible where Bitcoin is superior for 95% of use cases, but inferior in 5%, and the niche currency will be able to hold that 5%. The clones are nothing more than jealous haters who wish they were around during the Bitcoin boom.
I wouldn't say "impossible to add to Bitcoin" since that is basically never the case - you can add almost everything to Bitcoin. But as you point out, it may not be practical. I for instance consider Namecoin to be the most valuable "alt-coin" and of real use as opposed to most of the others, since it has some very real and great additions over Bitcoin. But while it can in theory do everything that Bitcoin does and even more, I think it makes sense to not use them "also" as a Bitcoin-replacing payment system (or try to do so) but rather have different blockchains for different purposes (payments with Bitcoin, every kind of securing/freeing "information" with Namecoin).