OK, so I am a complete newbie to leatherwork, but I have been sewing all my life.. including a bit with suede
If the pattern is using a heavier weight fabric like denim, thick wool, cordoroy, brocade etc then you will probably get away with a very garmet leather. You need to consider seam thickness, lining, and folds.. as all of these will handle/hang/wear differently when made with leather. You will probably need to not use any interfacing or stiffener if a pattern calls for it as most leathers/suede's are enough on their own.
If the pattern is for a light weight fabric like silk, cotton or synthetics... it will depend entirely on what the item is. But it would be a LOT harder to swap the pattern over.
You mention messenger bag.. so the short answer is probably yes.. it will work.. just check carefully how they have done seams.. you may need to alter a tiny bit as leather doesn't fold so easily or crisply as thick cotton drill does.
Hope that helps
Natalie