Lamp failures can happen in a $30 G2 or $400+ full blown weaponslight, there simply aren't any guarantees. But, with a 5.56 AR being a light recoil weapon you can have better 'luck'; we've seen many soldiers who apparently had no other option but to duct tape a G2 or 6P to their h.guards or FSB. Many reported using std. G2/6P's with no problems while a few others stated they did have a lamp failure, was it recoil related or just a simple failure??? Its simply hard to say.
Now for "Po I need a shock isolated bezel?" That has to be your decision, it's an add'l safeguard which is always a positive but if that expense is not in your budget then its certainly not a reason to go without a light.
Whatever you choose, the best thing to do is keep the light back from the muzzle and have it in a good, tight mount. Next thing to do is to burn a set of batteries through it, most lamp failures happen nearly instantly or within that first battery change. Try to protect the light from hard hits, especially when its hot, that can kill a bulb the easiest and shock isolation won't even be a factor. And last, you should always use reputable batteries.