Ok, I asked my car detailer yesterday about the clear bra.
Here is what he said in a nut shell:
Hard to get off later (took 12 hours on a different car with the same 3M product).
Will discolor with time.
Will protect but if the rock is large enough - or moving fast enough - it will go through the bra anyway.
Hard to wax around the edge of the bra.
For the cost you can have the front end painted twice.
Road debris has a different impact at 100 mph than at 75 mph. So if you do a lot of higher speed running it will help more.
Bra is only really worth about $500. Dealer markup is steep.
I'm in San Jose, California so the winters are no problem - we don't ge the salt or sand on the roads. (no sandblasting).
If you follow other vehicles closely (read especially quarry trucks) the bra will help.
My 1992 Miata repainted once (two years ago) had no significant problem for me. My problem was I over-waxed the cheap original paint job removing the clear coat.
I think those were the points he brought up - may have forgotten one or two - the beer was good though.
So, it will be a pass for me.