I'd say it boils down to a matter of personal choice.
I won't lie, the second Leopard goes gold master, I'm downloading it. No doubt in my mind.
However, the second it arrives on the Apple store online, I will purchase it.
I have 2 Macs in my house that are able to run Leopard. I will purchase a copy of Leopard and install it on the two computers. No family pack.
The way I see it, Apple makes Mac OS so that it can only be run on a Mac. That alone means that I paid more for the hardware. It's like the cost of the OS upgrade is built in to the cost of the hardware itself.
Just as Microsoft charges ~$500 for Vista Ultimate premium or whatever, Apple still indirectly charges for Mac OS. "But Mike, the OS that comes on the computer is what you're paying for, NOT the future upgrades!!11!".
Me paying about $99 for Leopard (student price) is good enough. Apple makes a killing on their hardware, and me installing it on 2 machines instead of just 1 isn't going to hurt. And yes, if everyone did it, Apple would still be in big business.
Am I wrong for downloading Leopard release candidate and purchasing it when it comes out? I don't think so. In fact, I'd even say I would be part of the beta testing team, along with anyone else running 10.5.0, regardless of it coming to you in boxed or downloaded form, so long as you have (or will have) a license.
My $0.02