The simple fact is that a merged mining pool is currently about 45% more profitable than an ordinary Bitcoin pool. Only a few pools have implemented it at this time due to the scrappy state of the code and the lack of protocol documentation and all of these pools appear to be passing on (or intending to pass on) the mined namecoins to their users.
Well no it looks like 350GH of hashing power is "missing". Either they are all solo operators or some pools are collecting namecoins without notifying their miners.
Most of that is slush and he fully intends to pass those namecoins out to the miners. Currently the merged mining is just testing. I'm sure slush has taken appropriate steps to ensure that the pool users are not being affected by this testing.