I have the fifth page on my Garmin 500 set up for "race". It has power, speed, distance, cadence. (Distance replaced HR.)
When I took off HR and found that I can push harder for longer. I don't look at it much during a race (not at all in a crit) but even that quick glance during a road race is enough to freak me out during a harder effort.
As to why the Pros don't use it... I'm watching Stage 9 right now, and I see five guys with jerseys unzipped, only one is missing a HRM. I know that's too small of a sample, but I'd guess more have them than don't. Power is the best metric for how you're performing, but having all of the data available after the race for analysis would be important. "Why did I feel like crap on that climb?" .... "Oh, my power numbers were only slightly lower, but my HR was up 10bpm." I remember an article back in the day where Lance would use a thermometer in pill form during training to get extra data to help optimize technique.