I know of some pools who are trying to implement merged mining. However ATM there are some perfomance issues that even MasterPool suffers from at about 50GHash/s for which I have workarounds in place but it needs to be fixed permanently. Not to mention that MasterPool architecture was designed to deal with this splitted blockchains stuff from the beginning. Everybody is working right now lowering the foot print of merged mining. You have to understand that big pools optimized their complete architecture for mining one blockchain. Now they have an additional blockchain, merged mine proxy and additional calculations in their backend. It feels like hitting a concrete wall with a jet   The problem is you CAN'T simulate large load easily on testnet. Thus I guess some pools are turning it on occasionally in order to make sure merged mining optimizations are working properly turning it off if they gathered enough data to do the next optimization iteration. So please don't panic and wait a few days.