I would have to say no. Although there may be huge demand for such a machine, the Frankenmacers would end up buying one and holding onto it forever rather than following Apple's upgrade path. You only have to look at all the beige Macs out there running Tiger for proof of that. If Apple officially allowed people to upgrade their old machines into inifinity, they wouldn't sell any of their new models.
That's what I think too. If apple sells you a mac pro, you may keep it forever, but at least they get a huge chunk of change now. If they sell you a mini or an imac, the margins are lower, but you have to upgrade sooner. If they sell you a midrange tower, they get a small profit now, and then you hold on to it for years and years, feeding money to 3rd parties for upgrades.
The reason to sell it would be if apple thought that there were people not buying macs at all who would buy this. Apparently they don't think that's the case, and I tend to agree. I think most people (not 100%, obviously) who want this tower will either settle for an imac or stretch for a mac pro rather than go the PC route.