I have been using HRM for years - first in running and now in cycling. For me, it not only shows how hard Im trying at the time, but also is a good measure of your general fitness level (resting hrm before exercise) and ability to recover following a climb, for example. I always measure against my history, not anything else. Max HRM Ive just guessed at, I see why its useful to have it, but I know roughly what it is just by looking at my data over time. You can use one of the sufferfest Rubber Glove tests to measure it.
On the cadence front, I never used to bother with this other than on the rollers, but in the last 6 months or so, Ive read, (and tried) that its generally easier on your body (knees and legs etc) and gives you better overall performance (especially uphills) to keep your cadence highish and stay on top of the gears all the time. To force myself to do this, I measure average cadence over a ride, and keep an eye on it when Im pootling along, to try and hit the figure I have in mind (usually around 90). Felt weird at first because Ive always had low cadence and just relied on muscles more, but now the shift means Im balancing muscles and aerobic strength (heart!) - seems to improve my fitness as well as get me wherever a bit quicker.
I agree if youre not trying to improve fitness or performance, you probably dont need to put on your hrm/cadence, but maybe it'll give you a warning when somethings wrong as well. (like cycling if youve got flu coming on)?