I think another factor is that disk drives have gotten so large compared to your average *nix that it doesn't really make sense not to include everything anymore on a desktop machine. Which is not to say that stripped down versions don't still have their place, but on a machine with half or even a full terabyte standard, tossing out the kitchen sink is more work than it is worth.
Though note that the server software is a custom install option; i.e., it's not there by default.
Also note that Lion will be sold from the App Store, so physical discs (DVDs) are probably out. (They'll likely include one of those little flash drives with the new hardware with the install disk image on it.)
