I think Namecoin has a good value, but while I think Vladimir overestimates the inflation effect of merged mining, I still mostly agree with his post, which is negative towards uncontrolled merged mining. It requires small patch for bitcoind to make it "parent" for Namecoin blockchain, is the patch exclusive for Namecoin? If yes, what makes it exclusive, and if not, is there a way to design a merge mining patch, so it would be exclusive for a particular blockchain? That way bitcoin community would have a choice, when downloading a new version of bitcoin, if it wants to adopt a new blockchain.
Your question seems to be based on a misunderstanding. The patch simply allows you to do merged mining. It doesn't add support for other people to do merged mining to Bitcoin, as that is not necessary.
Anything placed in the Bitcoin hash chain will be secured by the Bitcoin mining process. If I put "HAVE A NICE DAY" into a transaction, as Eligius puts prayers, they will become a permanent part of the hash chain and will be secured by all the miners currently mining the Bitcoin hash chain whether they intend to secure it or not.
All that is required is that it not be illegal according to Bitcoin rules. (Otherwise, the block would be rejected.)
The patch has no effect on the parent blockchain, it just allows the patched bitcoind to be used for mining on both chains simultaneously. Only the child blockchain needs to be modified in order to support merged mining. There's no way for the Bitcoin network to prevent merged mining, even if doing so were desirable.
Merged mining does have to potential to allow alternate chains access to greater hash power (and thus security against double-spend attacks) than they would have had otherwise. However, this alone will not give them value as currency.
Bitcoin is able to sustain value because of its network effect. The more active users it has, the more useful it is to each user. The alternate blockchains have userbases that are a a few orders of magnitude smaller than Bitcoin's. Newcomers to the cryptocurrency world will tend to choose the more useful and popular chain, further increasing Bitcoin's lead.
AFAIK the upcoming patch is for merged mining between Namecoin and Bitcoin only. In the future it might be able to modify the patch to allow more generalized merged mining between any number of coins.
This is not a bad thing
Quote from Gavin in the thread you linked to:
I don't see how shared mining dilutes bitcoins.
That's like saying "real-world miners find lots of other minerals when they search for gold; that dilutes the value of gold!"
Think of it as the bitcoin family of coins/blockchains. Eventually you might want to diverge your holding between the different coins, but the entire family just becomes stronger by universal merged mining.
Keep in mind that whenever an alternative chain enables merged mining it is centralizing a huge amount of trust in the bitcoin ("Nakamoto chain") mining pool operators.
When people join a bitcoin mining pool, it's generally because they want bitcoins. Sure, a few people will ask their pool operator to add thischain or thatchain mining, and some pools will. But plenty of pools won't.
Anybody running a pool who has not told its miners "hey we are mining blartzcoin and I will split the reward with you" basically has a gigantic pile of blartzcoin hashpower to direct as he/she pleases. They're totally unaccountable in how they use that power. If they use it to screw up the alternative blockchain, and in doing so they sacrifice the altchain mining rewards, none of their miners will get upset and jump ship because they weren't expecting those rewards in the first place.
Merged mining is risky for altchains unless you can be reasonably sure that miners will demand that their pools mine that chain. Right now I think Namecoin is the only case where that's even close to being true. It's also the only chain with a coherent answer to the question "wait, doesn't bitcoin already serve this purpose?"
The patch allows many chains to be mined at once. It does not let them automatically get mined though. A miner or pool has to still choose to run that chain and add it to its merged mining proxy. So the chain still has to get support to get access to that power and get installed. No one can make a chain and force it to be mined (unless they are a large pool operator).