I can see value to both approaches (listening with / without accompanying visuals). Certainly the listener's own imagination and the mental images, feelings etc. that the music can create on its own are the most important. Without that personal connection, there would be very little purpose or content in the whole thing! I feel that's true to enjoying any artform, whether audio/visual or otherwise.
With ambient music in particular, I think there are many layers to the listening experience, and there are potentially several ways to listen to an album. Sometimes I let my imagination wander freely, sometimes I enjoy letting the artwork play more of a role (and often it's a mix of the two). Often it depends on the time of day as well and where I am. The question of whether or not some image / video, or other "extra element" (such as text) fits the music or not is of course very subjective... and also depends on the type of artwork involved, whether it's completely abstract and textural, or something more concrete.
As for the marketing element, it is obvious that this plays a role for any musician who is making their living from the art they create. Regardless, I like to think it's possible to go an extra mile with this and do one's best to produce work that has some sense of soul to it, even if it's just part of the packaging or a promo video. Of course, when creating such things, I can't guarantee everyone will agree with the outcome... in this sense at least, it's a very similar process to music-making! My attitude towards this may also be "somewhat" biased, since I spend a fair amount of my free time in this area, for better or worse