I do this all the time, but none of my teams run man defense. I'd think carefully about doing it with man. With my zone/press and press teams, though, i'll often use a good scoring guard as the #2 at 2 or 3 different positions (especially if playing 3-2z/p). It can be a good way to utilize a scorer that has some defensive liabilities that you wouldn't start, and it doesn't allow the defense to key in on him without double-teaming.
I do this sometimes when starting freshmen, but also in instances where I'm trying to minimize underclassmen minutes from my bench. Say I have 5 guards. 2 solid seniors, one good junior, and then 2 underclassmen that need development. I could just use target minutes, but that has its own pitfalls. So in this situation, I'll often start my 2 best guards, put my third best as #2 at both spots, then set the underclassmen as #3 in their respective slots. Then I'll take a look at stamina of my two starters. The guy with the most stamina, ill set fatigue to getting tired, instead of fairly fresh. The guy with lower stamina stays at fairly fresh. So in normal circumstances, my super-sub comes in and plays a lot of minutes with both starters, while my underclassmen don't get overexposed.