I had a FitBit a few years back and used to monitor this for awhile. I did find it useful for gauging my overall conditioning over a period of time by checking rate for comparable climbs, weeks during my rare training periods, changes in resting heart rate, etc. These numbers did all generally correlate with a perceived feeling of improvement and actual better times on hikes versus book time.
I didn't find it terribly useful for a single hike. When I was exerting myself it was high. When I was walking easy grades it wasn't. I don't think I could look at it and say "Wow, 206. I'd better stop for a minute". If I was that gassed I would have already stopped because I had to. I didn't need the FitBit to tell me. I routinely walked at heart rates that common rough calculations would deem dangerous or "max" that were well within my comfortable limits without any worrisome warning signs.
I think a little common sense goes a long way. I've never had a heart attack or been in a group with someone who has but I'd expect the person having a heart attack was well aware that they were "pushing it" and either didn't want to hold up the group, admit they were not in shape for the task, etc. I think it's just human nature to press on even though warning signs are flashing.