I never do that, and it seems unnecessary, and waist of precious time until publication. Publication time is long as it is, there is no need to stretch it more.
I have tried this a few times and it's OK in general, usually the critiques either relate to length or specificity of the article and how it fits in with the general scope of the journal
I was recently at a meeting for editors of applied linguistic journals and most of them said they welcome submission inquiries. These usually focus on whether your paper is suitable. This may relate to the subject or some issue, for example, regarding prepublciation. Of course, if you know this, you don't need the letter