There are tons of things to consider, but i like to talk in terms of fun. Being unable to use attacks in a spec based on fast gameplay is not fun at all (combat), but having to pool energy in a spec based on buff uptime is fun (assassination).
In terms of fun, the jump from expansion end to start feels really bad, my point is not that we should be able to have as much energy regen as we do at end of expansion, but we shouldn't have to struggle with it as we do at start either. The middle ground is the right way: have enough baseline energy regeneration for each spec to make it feel fun and make haste have less impact on energy regen.