This is about a month-old post now, but as nobody has replied, I'll make a few comments.
As you're probably aware, "reflected" light meters presume middle levels of reflectivity. Dark-ish and light-ish subjects still throw the accuracy -- even today on the latest modern cameras. Cameras tend to over-expose their metering estimate of black (or dark colored low reflectivity) subjects (because they think the subject should be brighter) and under-expose white (or high reflectivity) subjects and we're just used to dialing the exposure compensation up & down when using a semi-auto mode (Tv, Av, P) or just metering for higher or lower then the center position of the light meter needle when shooting manually.
Incident meters are spot on because you don't point them at the subject... you hold them in front of the subject with the light sensor facing the source of light. So they really are measuring the light that's about to fall on the subject and not the reflectivity of the light off the subject.
BUT... you're right... often times we can look at the subject and compensate and still be within a third of a stop of where we want the exposure (and if you're shooting RAW, you have so much adjustment latitude that this is more than adequate.)
There are a few things the better hand-held meters can do even beyond incident metering accuracy.
The mid-level and high-end meters usually allow you to take multiple exposure samples and they'll compute the median exposure (not the average... that would be bad. It's the median.) If you've got a portrait being lit primarily from one side, you can meter the shadows and the highlights and the meter will report the median exposure -- so nothing will be clipped.
They _also_ are fantastic for metering lighting -- even manual lighting. Built-in meters don't know to watch for that "burst" that a flash strobe emits. But many hand-held meters do (all but the most basic of hand-held meters). So it can be handy for that. The slightly better hand-held meters also report flash exposure "contribution". This is great for mixed lighting scenarios (e.g. both ambient light and flash) where you don't want try to match the flash to ambient for a better look (for example fill flash). But usually you don't want them to be exactly even -- if they were exactly even you'd get a flat subject and if the flash is too strong then the subject will appear to be only lit by the flash. So by using a slightly lower power, you can have a subject primarily lit by ambient, but with fill to reduce the severity of dark shadows and turn them into gentle shadows for a more dimension look without being dark.
Turns out the mid and high end meters can do this "flash contribution" calculation and will tell you what percentage of light is coming from ambient vs. what percentage comes from flash.
Lastly, the very high end meters can "profile" the dynamic range of your camera. You take several exposures of a special target, import the images, and it evaluates the dynamic range of your camera and loads the profile into the meter. When you then meter a scene (usually looking for the brightest and darkest points in the scene), the light meter will tell you if it fits within the dynamic range of your camera.
