this is not a journal title

Well, different designers will have subtle variations on the height of the lapel points, or the cut of the cuff, but there are only three or four basic styles of tuxes, and you could typically put two designers' versions of the same style next to one another and a casual observer would not tell the difference. There is more room for variation when you get to the accessories -- bow tie, cummerbund/vest. But one other thing that makes tux rental easier is that all these things are modular. You can choose jackets separately from vests. And since the jacket and pants are separate, it's easier to fit a wide range of body types.

I suppose it depends on the nature of the wedding. Often the groom wears tails, which, in today's fashion world, are pretty unreusable unless you're, say, an orchestral conductor. At the last wedding I went to, the groom was a naval lieutenant, and wore his dress uniform. I don't know what I'd do if I got married, but if I were ever in a wedding party, I would probably wear my own tux pants and shirt, and rent a jacket and accessories if I didn't have the particular style. Granted, if I were in a wedding party next month, I'd probably have to rent everything since my current tux doesn't come close to fitting, and I can't afford to buy right now.