For the majority of musical acts, the songs and vocals are the most important part of the package, and the biggest selling points. Having a following and touring history are also key. That being said, a great image/visual presentation can also very important to an audience, and for marketing purposes (e.g., MTV, mags). I have found that industry types tend to prefer to see a group perform live before they can really decide if the band has the right stuff. However, if the band cannot get the industry to come see them in person, then having a video for the industry to watch at their convenience could be a good sales tool, and might help create the desired effect (although the energy of a live show rarely comes across on a tiny screen). If you do plan to invest in a promo video, I would not suggest spending too much time and money on it because the odds of it being a determining factor in an artist's career - or even being seen - are not that great in most cases.
Ben McLane Esq