I think the point is that unlike an HID, an LED light properly designed is nearly unbreakable, if you drop an HID on concrete it is very possible to shatter the bulb (or endo onto a hard rock, etc).
Also, an LED is dimmable, they are actually happier to run at lower power levels, and so you can run at 50% power for twice as long, still have tons of light, and no problems.
You can strobe them, they can start up instantly, umm....
They CAN be MUCH lighter (no ballast or heavy glass) but that depends somewhat on design, optics used, etc.
Oh, and if designed and used properly (not overdriven or allowed to overheat much), they are typically rated at a 50,000 hour lifetime, and that's just to a dimmer output (70 or 80%, depending on manufacturer)... if you take even 1/5th of that before a slightly noticeable drop in output occurs, you are looking at 10,000 hours, or the lifespan of 2-4 HID bulbs (at $80+ a piece replacement costs, IF you don't break them sooner)... in other words, an LED light will require no maintenance and will last "forever" in realistic terms.
All that, for the same price as an HID, which offers none of those benefits. The only argument left really would be beam patterns and color, both of which are variable based on LED used, and optics/reflectors used in the design, compared to a pretty standard HID pattern (only a couple of HID bulbs used in the biking marketplace, what you see is what you get). Really as of this moment, LEDs on a lumens/watt/dollar basis, are equal to or slightly better than HID in the commercial marketplace and (and can be much better if done by a DIYer). In 6 months as LED emitters are getting more efficient (10-20%), they will pass HID in the efficiency area to the point that HID will no longer make sense and most likely will quickly disappear (or drop substantially in price due to dropping demand, since they already have the tooling and R&D investments in HID designs, but since the bulb is a large part of the cost, the makers of the bulbs will have to cut price as well, and probably won't for a niche market like bike lighting).
Anyhow, that should hopefully give a good overview of "why LED over HID" in current market terms (equal light per watt, more feature capable, potentially lighter and smaller package, longevity and durability).