[Mithalwen]

I made and somehow lost a long post on this last night when I had got my copy of the letters and while Galin is correct that Baynes's work didn't universally get Tolkien's approval it is clear from the published letters that he did go into some detail with her such as the feather in Bombadil's cap. His letter offering the poems to Unwin express the wish and hope that PB will illustrate.
I don't have time to type out all quotes (again) but in his letters to her it is clear that he recognises the difficulty in the task and that she is making an interpretation - noone could perfectly illustrate the contents of another's mind after all. And in letter 211 to Rhona Beare he says "I do not know the detail of clothing. I visualise with great clarity scenery and "natural" objects, but not artefacts .." and then comments on PB's illustrations and gives other details which explain his own ideas as far as they went.
In his letter 260 to a composer he hopes that the result might be "akin to my own inspiration - as much as are, say, some (but not all) of Pauline Baynes' illustratrions".

Obviously I do not have the privileged access of Hammond and Scull but my guess is that Baynes was sufficiently in harmony for Bombadil where as representations of his creation they are "as pictures in a tapestry of antiquity" but when it came to representing the characters of the main published work he demanded more. The difference between portraying a "historical" character rather than a legendary one - if that makes any sense. Anyway the letter to Rhona Bear is particularly relevant to costume to get back to the original question.