I don't train with a HRM and am one of the heretics who doubt how useful they really are.

My belief is that its biggest value is to keep you from going faster than you ought to. It helped me I guess when I ran. When I MTBed it was worthless because I would very quickly peg out and could not avoid it unless I just dismounted and walked by my bike. For road biking its not very helpful either, not with all the rollers I do routinely.

I don't need a HRM to know when I'm close to redlining it. I just start to feel like puking and back off.

The racers probably find them helpful but for the rest of us it generates numbers to geek out about.