I had the pleasure of working with one of the best electrical (motor) companies in the states, and decided to sit in on the journeyman electrician course (because sysadmins want to know everything), and I was surprised to find out that DC is what is used for the large high-power long distance transmission lines. I think there are probably quite a few applications where DC might be more appropriate than AC that haven't been realized yet.
DC works for transmission because it operates at incredibly high voltages. Power is voltage * current, but resistive losses are proportional to current only. So the theory that makes it work is since P=I*V, energize the line at incredibly high voltages and get very little actual current flowing to minimize resistive losses for a given power rating. UHVDC is something like 800kV.
DC is much less workable at low voltages like 12v or 5v common in household application -- the P=IV equation is more dominated by current, so with more current the resistive losses add up quickly even in a run across your house (it'll be down to 10 or 8v by the time the wire reaches the far end of your house). Not to say some people haven't suggested wiring homes with 12v lines... data centers have implemented (or at least experimented with) DC distribution due to the density of DC devices.
The benefit of HVDC transmission lines is that you don't have the skin effect at all which makes a difference even at 60 Hz when the cable is as thick as your arm. If you've ever seen one of those large transmission lines where each phase has three conductors all very close to each other but kept slightly apart that's done because of the skin effect.
Also there's always capacitance which will cause some loss for AC but of course capacitance won't affect a DC transmission line. HVDC also makes combining different electrical grids extremely simple as there's no phase difference or frequency difference to worry about. DC also requires lower peak voltages as AC is going to be sqrt(2) times higher than the rms voltage.
AC powerlines start suffering unacceptable dielectric losses above 750kV. DC power lines can go to about twice that. I don't want anything near 750kV anywhere near me.
DC power lines are also better underground or undersea, they don't suffer from the capacitance to the surrounding material.
The AC-DC conversion equipment at each end has a cost, but for 300 mile aerial lines or 30 mile underground lines it is currently cheaper to do DC.
I'm little enough of a hardware person that I'm just interested for the reasons the OP mentioned; constantly switching between AC and DC and AC and DC is obviously inefficient, and batteries are expensive enough that it's probably cheaper to add a DC circuit than to double or triple the battery capacity.
Tesla's powerwall claims 90% round-trip efficiency (AC in vs. AC out) and most SMPS power supplies are also 90%+efficient as well so you are losing <20% capacity (not 100-200%).
