All you have to do is compare a tube that has gone the distance to a new one. I am talking about a tube that was installed in a new tire and stayed in there until the tire was worn out. The old tube is a pathetic, drooling flaccid shell of its former self, while the new tube is firm and confident, gleaming with its new coat of talc, exuding the air of something that can prevail over anything the road can throw at it. I know which one I want inside my new tire.
All seriousness aside, as always, it depends on the circumstances. Are you putting the tube in a new ME88 that will go 15-20,000 miles before it is worn out? Better put a new tube in there. Is the bike going to be ridden 500 miles a year only on bluebird days for the forseeable future? Then by all means, cheap out, put the limp old one back in there and pat yourself on the back for being so green. I'll still pull over to see if you need any help with your flat tire if I see you out on the road.