The most important factor for any currency is security.
That usually means if they aren't able to be merged mined with bitcoin they are doomed to be attacked if they ever become worth attacking, since ultimately the security model for a blockchain is that even if everyone in the world who is not securing the chain with their computer power were to attack it, they would all add up to not enough to threaten it because it has way the heck more computer power securing it than the entire rest of the world has.
So really bitcoin is the only secure coin, even namecoin is not as secure as bitcoin.
So if we want more than just bitcoin, we really should start by making sure the merged coins, like namecoin, are in fact merged by pretty near all the miners who mine bitcoin, which means basically ensuring all the pools do merged mining. potentially if all the pools merged all the coins that can be merged then possibly we could end up with more than just bitcoin and namecoin as reasonably secure coins. But is namecoin reasonably secure yet even? Hmm...
-MarkM-
Bitcoin has never been attacked by a serious entity...
Only by relatively small, illegal botnets for short periods.
This gives BitcoinLand an overblown sense of security...
A group that has one (1) paid Dev... and works at a snail's pace with 2009 technology.
A government attack = arrests, tax audits, exchange shutdowns, disinformation campaigns...
Masses of people behave herd-like...
A concerted government action would create a spiraling loss of trust in the network.
It may not be just USA to worry about... China or Russia would be much more ruthless.
In China, for example, organizations must apply for a permit to use encryption...
Any encryption technology on your laptop must be declared as you enter the country.
So tying all crypto to Bitcoin may not be the way to go...
Also maintaining a monopolist position for BTC seems cultish...
There is nothing stopping 1000s of crypto-currencies being issued by every conceivable group...
We are really only months into the Crypto flood that started in early 2013.