The only purpose of bump helmets if if you need blunt force protection. You need this for stuff like if you are riding a bike, motor vehicle, etc. where you have a high chance of crashing into something hard. Although with the newer gel suspensions, even this is largely obsolete as many of the newer pads provide almost identical protection to a bike helmet. If you just want light weight, most current generation helmets that are IIIa and not rifle "rated" are as lightweight as any bump helmet on the market. On the high end of this type of helmet is the MTEK flux, on the lower end are Delta 4 (which are obsolete due to the Delta 5, so you can often find these in surplus for the same price as most bump helmets). These ultralight helmets are also smaller than a normal helmet you would be issued in the military which may make you think they are one size too small if the mfg label isn't present. The reason is they use a more rigid composite to save weight. Side benefit is your gear is a lot closer to your face so you aren't fighting as much inertia when turning your head. That being said, just wear your helmet more while keeping a good posture, and your neck muscles will eventually compensate.

I have both and generally run the bump because my neck hates me. As for a pound weight difference, I haven't seen that to be the case, my team Wendy bump compared to my pdg arch gen 3 is worlds apart in feel and weight. That being said I don't have a $1000-$2000 fast helmet.