This is mainly a hold over habit from my days training in high school and college. I keep good tabs on it.
Fairbanksdiver, yes, sometimes an athlete will target a specific "zone" of acceptable heart rate in order to keep their workload up. Heart rate "ceilings" are also used as an indication of athletic error. For instance, a well trained athlete knows that where his heart rate should top off during a 400m. If he/she goes over that well established mark, they know that they are not staying in form and have lost stride efficiency. That might trigger some video time with a coach to regain lost form. Anyway, that’s just an example.
And if you think monitoring your heart rate is not an easy way of monitoring your SAC (an inelegant formula and concept), I don't know what to tell you. The bottom line is that if you can keep your heart rate down (by identifying the times that it tends to rise) you will keep your SAC down by proxy.
More to the point, I take it you don’t know of a computer that has this?
Thanks,
JB