I think too you have to look at the fact that, an in game DPS meter would actually not tell you very much information.

This is why parsers have a lot of different stats that they show you, because you need to analyze the data to actually understand what is going on.

The DPS number itself is often times the most useless piece of information.

From a Wow forum "For example, one of the big differences between Recount and Skada is how they calculate the "per second" part of DPS. Skada uses the time of the whole fight where recount uses the time when players are active. This is a decisions that Blizzard would also have to make and there isn't really a right answer.

The key to the DPS issue is not having an official meter but developing, as a community, a better understanding of what a DPS meter actually means. Far too often people are taking extra damage, ignoring interrupts, and screwing up mechanics so they can chase higher DPS numbers. If we, as a community, agreed that DPS is not the single holy number that all must chase we would be better off. There is far more to playing this game well than just doing more damage."