I was raid leader throughout a good portion of wrath as a disc priest (tail end of Naxx through to end of ICC) - I found it easy enough especially in terms of seeing individuals with buffs/debuffs, and usually if there was a damaging ability coming up I would know about it anyway due to preparing to react as a healer. I also felt I had a good general overview of how everyone was positioned. I did switch guilds in time for cata and our current RLs are usually a prot warrior or a dps dk - both of which do a fantastic job.
At the end of the day I believe a lot of what makes a raid leader good is not the ability to shout out during combat - but the ability to explain with both clarity and suitable detail level before an encounter in such a way that the raid team is capable of executing the strategy. Any class can do this, and maybe it is just RL personalities affecting class choice that leads to the trends seen.