Also there is decreasing reward as more people "do it".
NMC is worth about 0.01 BTC. This is w/ roughly 1/8th of Bitcoin miners doing merged mining.
As more people merge mine the difficulty of NMC rises. If hypothetically 100% of Bitcoin miners merged-mined then eventually difficulty of NMC would equal BTC.
Currently the ratio between BTC:NMC difficulty is 7.69. since 1 NMC is ~1% of BTC that mean merged mining provides a 7.69% "bonus".
However even w/ next difficult change that ratio will drop to 5.81 which at 0.01 BTC per NMC is 5.81% "bonus".
If price remains ~0.01 BTC and 50% of miners merged mined then the bonus would be ~2%.
Each additional pool gains "less" for the same amount of work. When it first started the NMC "bonus" was roughly 140%. Now it is ~7%. In a couple weeks it will be 5% and eventually could be <2%. Many pools may not see enough reason to implement it.
In all fairness, I don't think the main reason MM was brought about was to increase mining profits, but too ensure the safety of the Namecoin chain by putting as much hashing power as we could into it.
I for one love how Bitcoins design has enabled a decentralized TLD system, and think it too will be important in an ever changing government controlled internet.