why should a professional own both an incident light meter and a reflective light meter?

Answer 1

I only use one light meter, it's an Ambient Lightmeter by Gossen (the Gossen Digipro F).

I use it for all of my portrait photography and have had no problems or situations come up where I've needed any other type of lightmeter

Answer 2

Only if they need them. A photographer working with tricky lighting situations or when there is no chance for a second shot would do well to meter as much as possible. This of course was far more important with film then digital for several reasons. The first is that most pro digital camera have a histogram where once the picture is recorded the exposure can immediately be checked. The other reason is the greater latitude of exposure range of most pro digital cameras over film. (for a shot taken RAW format and at ISO 400 or lower, exposure can be adjusted almost 5 stops in either direction in most cases)

Answer 3

Measuring the light source is different than measuring the reflected light from the subject. Surface color and texture have a tremendous bearing.

You can certainly get by with one or the other, but if you want to be precise, use the right mode for the right situation.

Answer 4

It is not necessary to own both. There are light meters available that combine both features, and they are not expensive. Check with you rlocal photo shop and talk to the experts about what you need.