If the traders are arbitrating prices between different ad exchanges, their net price will be zero and they make the ad market more efficient. Who exactly is hurt?
If these arbitrageurs are keeping all the prices in different exchanges in line, it means that your average ad seller/buyer does not need to spend time and money on researching which exchange has best liquidity (although they probably still want to know which one chargers lower comissions).
Of course, there will be an effective fee that the arbitrageurs collect, but competition amongst them will drive that feed down (in principle, to less than what the average person would have spent on research).
They are matching buyers and sellers of ads across different exchanges, how are buyers and sellers hurt by this?
Yes, the traders pocket a spread for their trouble, but they also save you the hassle of checking all the different exchanges. And if you really care about that spread, you still have the option of looking at all the different exchanges.
At the end of the day, unless their participation increases x by more than their cut, they're just taking revenue from everyone in the game who actually provides value.
They don't make the market more efficient.