Namecoin data (unrelated to bitcoin) is added to the bitcoin block chain, thus making the bitcoin block chain unnecessarily larger.
Technically true. I agree that those things are literally incorrect, and when you're trying to make an argument on a contentious issue, it's good to be precisely correct. Merged mining does add some bytes to the bitcoin block chain. However, lots of other things add many more bytes to the bitcoin block chain and are of much less value.
In fact, if I were designing a Bitcoin replacement/update, one of my solutions to the reducing mining payout over time would be transaction fees for securing things in the block chain -- with an efficient "secure these hashes" transaction and a per-hash transaction fee. (This transaction would have no outputs and clients would know that they have no need to store it as a transaction. Block chain pruning could discard it entirely.)
In any event, if the transaction fees don't accurately reflect the costs associated with committing bitcoin transactions to the block chain, they should be raised. Unfortunately, there's a somewhat fundamental tragedy of the commons here. One person gets the transaction fee just for including the transaction in a block, but then everyone must store that transaction forever. This is a design flaw in the Bitcoin protocol that will likely get resolved in the future.