There's nothing inherently wrong with an "out-of-grid" vertex, assuming you mean that it doesn't lie right on a grid intersection in the views. It's just a vertex that doesn't have integer coordinates. They don't "do" anything different than a vertex with integers coords. The compiler will handle them fine. You end up with vertices like that when you make arches, etc., and, oftentimes, when you do a clip at other than a right angle.
The primary problem is what you already know - it's a pain to align brushes with one another when vertices aren't on grid intersections. It's easy to get alignment errors and not notice them.