If the bosses are tuned around optimized DPS and HPS, then a damage meter is neccessary to determine if you are participating at a level that allows you to down a boss.
It's not rocket surgery.
With no combat log and figures showing your DPS, the majority of people are swinging wildly in the dark. Not many people will pay for a game that they can't down at least a handful of bosses in.
I'd personally like to see a combat DPS/HPS meter that turns into a threat meter that shows personal DPS/HPS and TPS when combat starts. That would be the most practical use.
EDIT: That is to say it shows raid/party DPS/HPS meters out of combat as well.