The problem with scripting plot style is its vulnerability to idiocy. Something goes the way of the toilet when you're doing regular scripting, you've only got yourself to blame and you can learn from it and not repeat the error. When you're working plot style with an artist, you're putting all your trust in that artist, as well as most of the responsibility for getting the story right. Not to say, of course, that the artist cannot handle that responsibility; however, there's no reason for you to believe it. You know yourself, but you don't know the artist.
Just as art and anatomy are complex patterns to master, much the same can be said for writing and storytelling. Some writers spend all their lives seeking the perfection of it. A person who apparently needs a writer, logically, isn't likely to be a master of the form. Especially in serialized story telling, a la comics, if one wants to tell an ongoing story, a lot of continuity and relation has to be carried over from one month to another. Little references, thematic statements, etc, unless the artist has done as much background work as the writer - outlines, character guides, thematic planning, and the works - it would be almost impossible for him not to miss all these things in the construction of the story, leaving the writer only with dialogue to include them. Resultantly, either something is sacrificed or the dialogue begins to take a wooden and out of context feel at times.
Lastly, speaking for me as a writer, aesthetic integrity in the story is very important to me. Unless I know the artist really well, I'm going to be uncomfortable surrendering that kind of power. Basically, I trust myself more so than another. And I prefer to blame myself too.
That saying, it should in no way be doubted that the artist is a crucial collaborator in any successful book, that good communication is imperative, and that he understands the visual realm and what he can do with it far better than I. Therefore, when I did my comic project my methodology was one of adaptation and adaptability. The first few script I wrote without an artist and did a lot of playing around with descriptions and tricks. When the artist came on board, I studied how he was responding to different descriptions and what visually were his stronger and weaker traits. Than I changed the way I write to accommodate that. That has since had a profound influence on the way I approach comic scripting. Typically now, I describe by Panel, but only in general action (Character does so and so). I do not give layouts or angle directions. At most I will specify "camera" distance, with very few exceptions otherwise. Occasionally, I will mention if a Panel should have prominence on the page, but I've found it unnecessary. The rest of the Panel, by far the majority of the description, I dedicate to the mood and the mentality of the character which I wish portrayed. Oddly, most artists who've seen this method have disliked it, yet produced results far more in accord with my vision and artistically stronger than was the case with different approaches to scripting. Usually, the dialogue I write into these scripts is rough, and I rewrite it once the art is available.