A 60W headlight bulb is only pulling 1/12th of a HP from the engine through the alternator. Saving power is not an issue.
That leaves you with cost, quality of the light, and reliability.
You cannot put an LED directly on 12VDC. They are current driven devices, not voltage, so the headlight needs a current driver circuit to ensure it will not burn out when the battery cycles from 10V while starting up to 14.5V while fully charging right after a start. The circuit is inside the light bulb, where it gets very hot.
Only disadvantages I've noticed with led is the harshness of the light it give off
this is also an issue. LEDs are a point source of light, not like a wide filament. On many many cars now the tail lights are LEDs and they are the naked LED pointing backwards. Get behind one at a red light and the spot-intensity of the LEDs causes a persistance of vision etched into your retinas for well over a minute. Its as if you looked into a camera flash, you see the brakelight dots everywhere you look, even in the daylight. At night its worse.
To not use the LEDs as point sources they have to be pointed backwards into the reflector.
The other really annoying thing about LEDs on vehicles is to dim the brake light down to the driving tail light brightness, they pulse the lamps. I find this really distracting at night. When you scan your eyes across the highway side to side, you see the blinking tail light of the car in front of you drawing red dots all over the place in the dark night air. They could dim the tail lights by using a variable current driver circuit instead of pulsing them, but that would cost 85¢ more per LED... so they dont.