Namecoin WAS NOT designed as a vehicle to profit from, or to be easy to mine or even to be used in exchanges.
It was designed for the sole purpose of being an ALTERNATE DNS system and that's all.
Which still requires things like exchanges and nice GUIs. Even being able to access .bit domains is very user-unfriendly at the moment; actually registering them is positively painful.
Well technically, namecoin should have no value at all anyway.
It costs nothing to mine it if you do merged mining - i.e. it is free.
Right now, the software available for merged mining and the pools that support it are both a lot less tested and more unreliable than just mining Bitcoins, so there's definitely a cost to merged mining. At the moment I'm not sure the Namecoins I'm getting from MM are enough to compensate for the extra stales I'm seeing over a good BTC-only pool. Then there's the cost of developing and testing and setting up that software.
The negative side is (what I've said many times) it puts extra data in the bitcoin block-chain that has nothing to do with bitcoin
36 bytes of data per block - far smaller than even the smallest Bitcoin transaction. If you do just one test transaction you're bloating up the blockchan a lot more than merged mining. (The average Bitcoin block is somewhere in the 20-30 KB range these days IIRC, and that's going to increase if Bitcoin takes off.)