1. Miners will devote electricity and time to the most valuable line of work. If one chain is even 1% more profitable than another, all resources will be thrown there if possible. Today we have some guys with GPUs and cheap electricity, so they mine some litecoins during speculative game. But Bitcoin will continue growing while all altchains will remain hobbies or simply die.
2. Users will always trust the most powerful chain. Be it for timestamping monetary transactions or any other transactions.
You know about merge-mining, right? That allows one to mine bitcoins with one's electricity cost, and at the same time without (or possibly with negligible) overhead also get namecoins as bonus reward.
Merged mining is just a fancy name for "a protocol on top of main chain". I can equally start advertising my timestamps entered in main chain as "blocks" on "altchain" that is "merge-mined" with Bitcoin. Except that my chain would be much less costly, but produce the same functionality.
Of course, merge mining builds namecoin "on top" of bitcoin; and as you say, that is a good thing because it increases the security. However I still believe that there is a point in namecoin as a separate blockchain (although if you like built "on top" of bitcoin by merge mining). I do not know the technical details of how merge mining works on the protocol level, but I think it won't affect the bitcoin protocol but instead only the namecoin one? If so, then indeed namecoins serve the purpose of providing additional features (including easy ability to timestamp as well as a decentralised DNS) without diverting the bitcoin blockchain from its main purpose, and also without bloating the bitcoin blockchain. Even if one does not care about that, at least when the blocksize limit will be reached and free / low-fee transactions no longer feasible in bitcoin, it will even make economical sense to do non-payment transactions in an alt-chain which is secured by merge mining but not yet out of cheap block space.