Hi Kirk, yes, you can select and drag multiple items in e.g. LightRoom just fine and that actually works exactly as in PM (make multi selection, let go of mouse button, click in selection, drag). However, what in e.g. LightRoom is also possible is to select a single image (outside the current selection, if any was active) and immediately drag that. Better, this dragging leaves the current selection (if there was any) intact. Quite nice if you're used to this, I think (and it is the same behaviour as in Finder).
Like described, PM on the other hand does not allow this and clicking an image outside the selection immediately kills the current selection and drag results in growing the selection. Hmm, so I guess therein lies the problem, drag on a not-previously-selected item is interpreted as grow selection. The normal (?) behaviour in this case, however, is to drag the item instead.
This never bothered me, but now I'm thinking about it, I think it would be better if PM followed the “normal” behaviour here too. Perhaps add this as an option (though to be honest I think the contact sheet options to change the behaviour of the modifier keys is very much redundant; I can't think of anyone who would change the meaning of the keys to something other than what is normal for the OS (i.e. cmd toggles, shift extends).