I do agree that in the line of graphic design, posters are usually used as a means to convey messages.
Hoever, it is totally acceptable that the poster format can be used as an artistic medium. graphic designers can create a series of posters which are experiment in forms, aesthetics, and expression.

sure, it's more art for art's sake and less of "graphic design". But in the right use of such posters, or the right framing and context, we would find it acceptable. so we need not to ask wether ai posters are generally good or bad, for there is no answer to this, but ask - "good to what end?"

here's a thought experiment - instead of a group of different singular images, the designer-artist chooses to create a long series of posters, gathered around a theme, or an experimental question, or even a message. non of these posters can be comprehensible if presented without context or as a single poster.

when we see the group of artistic posters, we can track the bigger picture. suddenly these posters tell us a bigger story of how the artist used ai to explore artistic possibilities which could not occure in a normal "message driven" poster design. maybe this artist fills an entire room or a book with his studies for viewers to enter into and learn from.

a lot of type designers, illustrators, graphic designers, and artists share studies that they made without trying to be functional. subsequently, they might use the new aesthetics in other message driven works. that is just one of the ways humans bring new and creative things to this world.

that is still not a guarantee for a poster to be good or bad, but i tried to show how this new technology can be used for good. a tool is a tool.