The first clothing I made was by modifying existing items - I can recommend it for the reasons you give. You can see how the experts did it. I've seen pockets done both ways, both extruded and "patched on". The patch method seems to give better definition at the seams, but an extruded detail can work well if you run a very small bevel down the joint. To some extent, what you do will depend on the modeller you're using, and what its capabilities are.