It does not give the artist as much useful information as a live sitting.
When an artists sets up a portrait reference photo he/she may be looking for a specific color light to make the portrait more interesting. Yes, light has color, just look at a sunset!
The subject may be posed to create distinct shadow patterns that are both flattering and help create a sense of volume and space in the final painting. Raccoon shadows around the eyes, mustache shadow patterns under the nose, or a face full of flash destroying all the depth creating shadow patterns, are not great photos to work from.
At times it is necessary for an artist to work from old photographs only. I believe it is most difficult to get a great portrait with only these as a reference. The old photos always seem to be lacking somewhere and the artist must either make it up or find some other reference. Inferior reference material will require an extremely skilled artist to make something good of it. A reputable artist will also want to know who took the photo and if you have been given permission to use it by the photographer.
A recent flattering photo taken by a skilled photographer may look beautiful as a photograph and still not translate well into a painting. Many of these photos have weak shadow patterns or use a true white colorless flash. Photography has come a long way. The use of photographs as additional reference material is valuable, but I find the photographs less reliable then an artist’s eye in capturing some of the subtle color changes in skin tone. Just think about the difference seeing a breathtaking sunset in person verses a photograph of the same.
So while a skilled artist can do a fabulous job with just a photograph, he/she will be relying on his many other sittings and experiences for reference. If at all possible a live sitting, even a short one, will give the artist information that is truly unique and specific to your portrait and in this artist’s opinion well worth the time and effort.