The pragmatic and technical tradeoff is essentially that the proprietary drivers interact well with the GPU but don't interact well with the rest of the linux ecosystem (they are significantly more crash-prone, have many quirks which can't be fixed and need to be hacked around, and generally do their own thing instead of using established APIs), while the open source drivers interact well with the other software in the ecosystem but don't interact well with the GPU (such as the power management issues you mention, as well as lower performance). How big these problems are varies greatly from person to person and machine to machine.

The proprietary drivers likely won't ever interact well with the ecosystem due to their closed-source nature (although the situation could certainly be improved), while the open-source drivers could do better talking to the GPU if they had specifications and not just reverse-engineered info.