I think people who have aphantasia access memory differently than those who have not. And maybe dreams have less to do with memory, who knows. But thinking about it, my dreams might be lacking some things, such as less of a sense of touch and I also don't remember if I ever smelled while dreaming, I certainly do not smell in dreams anymore after losing my sense of smell, but that is a different story.
Your point about the "sanity/clarity" aspect is an interesting one, but it's not like this is guaranteed. Things like paranoia and similar mental states still can happen, and you can still have hallucinations. For example I have peripheral movement hallucinations quite a bit, where I think that something in my peripheral vision is moving around, some insect or what have you, but a lot of the time it is nothing at all. Something I have gotten used to over the years.
When it comes to art I think maybe it helps in certain ways. For me the process of making a picture is very much a journey of revealing the image to myself, a lot of the stuff I do is entirely subconscious and I don't really have a lot of insight into what I am doing from a rational standpoint, as in I could not really explain it super well. This however makes making art for me interesting. It is a way to externalise things which I have no other access to, without being creative.
I am not sure how much it helps in terms of not daydreaming, I never was the best when it comes to time management and procrastination, and I tend to think a lot about stuff, just having an internal dialogue. But once I get down to do something and get into it I have a very good focus and learn things quite fast usually. The matter is more actually getting to that point.
I don't think that this is a disability, but I don't think that the term condition is problematic here, because that is what it is. However, I have had bouts of depression in the past and the fact that the inside of my mind is void of images, sound and so on, can give you a pretty powerful feeling of emptiness and disconnection that is hard to describe.
Also things like being able to visualise my girlfriend, who lives in the Philippines, or other loved ones, relatives and friends who have died, would be nice. I know what someone looks like, and I have a very good facial memory in terms of recognising people. But describing someone outside of really hard facts, like eyecolour, where they have moles, etc., is pretty much impossible for me, and I doubt that I could draw many people from memory, if any.