It works fine. I've got about 10000km on a similar set up (7 of 9 + bb lockring on a 7s uniglide) and there are no issues. If you have the uniglide spline pattern it doesn't take too much effort to modify the splines on hyperglide cogs to work.
As for why, I did it because I couldn't find super-tight gearing (15-21) in 7 speed anymore.