I reckon hybrid will make sense in all forms of transport when
a) power density increases. LiPo is getting there, but the high density stuff is still in research labs, and a way off yet, and
b) when power generation comes from something much smaller than an engine, such as a fuel cell, whether methanol, hydrogen or lpg powered
I think pure battery power is a way off, partly due to the energy density mentioned, but more specifically due to the recharge capabilities. Even where batteries have been designed that can take a huge charge current to recharge quickly without boiling, the power infrastructure to get it to the charging point is not there. National grid would have to redig everyone's driveway and build new substations to make it possible!
Having said all that, I can imagine a day where a polymer based battery is used as structural components to the aircraft, as well as power. At that point, mass and power density take on a whole new meaning, as you are replacing more than just a power plant.
As for regenerative braking, it really isnt all that worth it! It helps a little, but I cant see it ever being of true value flying. It works for cars since they spend most of the time with very light loads, need a little push from the electric motors sometimes, and spend a lot of time coasting downhill. For boats, they are worth it since the wind is moving the boat, so you might as well tow a generator and get the water flow to work for you, but we are talking tens of watts, not hundreds or thousands.
From my limited experience of aircraft so far, except for gliders, when you turn the engine off or to idle, the thing heads down pretty darn quick. Even if you could generate hundreds of watts of power during that time, what you going to use it for?
Interesting discussion though!