The voltmeter can give you early warning if your battery is going south, or if there's a problem with the reg/rectifier. And if you are installing a bunch of accessories such as driving lights and heated grips, you can use it to tell when you're running too much and oughtta turn something down/off.
I've installed this Signal Dynamics voltmeter in a couple of past bikes:
http://www.directlineparts.com/product.asp?pid=4354
This one is just a single tiny LED light. It can flash red, solid red, yellow, green, flashing green, and each of these means something about the voltage. Green is good. Flashing green is overcharging, yellow is marginal, red is low, flashing red is dangerously low. There's actually a little chart in the box that tells you the specific numbers for these, and they're all very close together, but I don't have the number memorized, sorry.
It's been great on every bike I have had, except the TDM, which lives in yellow or solid red territory anytime the bike is idling, even if just for 30 seconds at a traffic light. It'll alternate between red and yellow, in time with the turn signals, sigh. Having this light show when everything is normal, is distracting. (And yes, it is normal. We have two strong running TDMs here and they both do it, they've done it for years, and neither has ever had another symptom of an electrical problem.)
So the question is whether the S10 is like the TDM and runs at a lower voltage as a normal thing, or is it like every other bike I have owned and the color ranges will be correct? I've already got one sitting on my workbench, so I'll probably install it just to find out. An actual gauge has advantages, but it's a whole lot easier to find an unobtrusive spot for a single tiny LED light.