Corporations bought site licenses because they were a less expensive, more flexible way to manage a lot of PCs -- at least as long as the PCs could do their jobs. The problem comes when the hardware inevitably falls behind the software, when a PC is just too old, too slow, its hard disk is too small -- and, particularly, I suspect, when Microsoft brings out a new operating system that obsoletes most of the hardware in the shop.
What do you want to to bet that the MAR program is primarily a public-relations program -- that Microsoft is trying to find a way to calm down some really big customers who are really, really angry about how much they face spending for new PCs that will run Windows Vista, while their current hardware inventory has been rendered worthless -- even worse than worthless, because the machines can't be resold with an operating system installed.
The MAR license is interesting for a couple of reasons. It's for XP Home or XP Pro, for one thing, not Vista, so its lifespan is limited. But then, it would have to be XP, because the reason these PCs are being refurbished in the first place is that they won't run Vista. And any refurbishment that would bring them up to Vista-ready would probably be as expensive a new PC.
Microsoft is in a sort of Catch 22 situation: it might have made more points with its customers if it had offered to pay the recycling costs of PCs covered by site licenses -- but while that might sell some Vista, it could cost more than it would pay. It could have announced that as part of the MAR program it was extending support for XP for yet another year -- but that would have reduced the pressure on corporations to upgrade to Vista, which Microsoft sees as an absolute necessity. I liked the suggestion of one of the commenters: The most effective program might be for Microsoft to offer corporate customers a free Ubuntu disk to install on every old PC it takes out of service to replace with a new corporate desktop running Vista. It's logical, but somehow I just don't think it sounds likely.