I do that. I'll smoke it down till it's getting resistant to burning, then once it goes out I dump the ash and put fresh baccy on top and relight. I figure if there's a lump that's going to sit and not burn it might as well stay there and act as a sponge for the next round. Doesn't seem to affect the flavor much at all. I only do this if the pipe's getting low and I want to smoke a bit longer - like on a road trip. I wouldn't try it if the pipe's been sitting around with stale baccy in it for a couple days.