AWay,
If you'll notice the next time you watch a road race, all those guys/girls will be spinnging at a fairly high cadence (say 90ish). It has to do with keeping yourself consistent while riding and makes for a good gauge of the effort you are putting forth couled with a HRM (obviously, there are plenty of other variables to consider like gearing, hills, etc.). Trying to maintain a consistent cadence without a meter is basically impossible (at least to me).
I have a separate heart rate monitor from the computer as I have more than one bike, plus I'll use it for walking or running or whatever. I did buy a HRM with the capabilities of basic computer functions (speed, distance) that I use for the MTB. I could, if I wanted, add cadence and power to it, but don't really care about it. (I might someday add the cadence, but it's not a priority for me since I have a seperate computer for the road bike).
I know that I've kind of oversimplified it, but hopefully that will help.