I've reused both lump and briquettes and I don't think it's a problem. Like somebody else suggested, I just put some fresh at the bottom of the chimney and then fill in with the "once fired" on top.
This winter when it was a "freezing" 20 degrees down here, I cooked some butts overnight using a "smoking jacket" to insulate my WSM. I couldn't believe how much charcoal (K briquettes) was left after a 12 hour cook). I used that with about half a chimney of fresh lit to do another overnight cook - again with the smoking jacket - the next week. I didn't seem to suffer any ill effects.
Now, like when I was shooting skeet - I would use my reloads for everyday, but come tournament time, I broke out the fresh stuff. I certainly would do the same for any competition, if I competed (someday, another story).
Just my 2 cents worth. And, oh yeah, I'm kinda cheap so that might have something to do with it, too.
Pat