It could be the switch, depending on the internal design. Doesn't seem logical, though. I would figure the switch would probably be setup like:
_ _ on
. _ park
. _ off
Where the lines are contacts and the dots are...nothing. So only one circuit is completed at once, and shorting to on between 'park' and 'off' shouldn't be a possibility. But who knows. Older cars are more prone to switch issues, simply because the switch needed to be more complex. But since your car is new enough to have HID's, the lights are probably all managed by an electronic module (BCM or the like) and the input side can be simplified.
Point being, if it's the switch, I would bet it's one of the other functions that has a problem...like the 'flash to pass' being too touchy. Try wiggling the stalk around with the switch in all three positions and see if you get a flicker.
Otherwise, doesn't seem like there's a lot you can do. I would check a similar vehicle (your model year or close, pref with HID's) and make sure it's not just something they all do. I could especially see this being possible if there are auto lights that may come on with the switch in 'off'.
I don't think it would be a relay problem unless it has one of those funky relays with two coils and two switches. One switch could be getting bumped closed momentarily when the other coil activates/deactivates.
FWIW, though, I don't think you're decreasing the life of the bulbs. There's no filament to break; it's an arc. I believe the amount of time the lights spend turned on is far more relavent to bulb life than the number of on/off cycles. There is also the issue of the ballast, though; not positive how that works. I wouldn't think it would have juice all the time, but if it doesn't, it seems like there would be a delay in turn-on time.
Oh, and 100 bucks a bulb seems like dealer price. I think they've come down to maybe 50-60, tops. Still pricey, I know.