"I usually use Modulation to control dynamics, with Expression used to refine the extremes" -Paolo
Yes that's exactly what I meant. It would be unnatural for example to have loud brassy sounds that were low in amplitude, and since expression CC11 controls only amplitude it is strictly for those extra touches - usually having a pianissimo note tail off farther than the note actually goes down to with the velocity layers, or perhapos to come up on the attack of a note out of almost nothing.
One thing I thought of that was unclear in what I said was I didn't mean to use velocity crossfade/modulation on dynamic samples which should always be just note-on velocity otherwise it is totally weird and unnatural. But you knew that. Though throwing in some expression with those is often really good.
Also, I have never liked - though I know some people do - using velocity crossfade with short notes like staccato, because it does two artificial things - blends separately recorded samples for the entire duration of the note (not just a fade), and doesn't actually reproduce what a player does in real time, which is hit a note at a single volume without any change throughout the length of the note. This applies even to somewhat longer short notes like detache or portato. So on those short notes, as well as the dynamics, they should always be note-on velocity with no crossfade, and the continuopus notes only with crossfade. One additional benefit of this approach is to allow sudden contrasts when you switch from a note-on velocity short note to a crossfaded long note.