I'm a bit late to this party, but I'll cheerlead a little anyway.
It's already possible (and easy!) for a user to get the benefits of both models. All you need is the right distribution. And that distro is GoboLinux.
Software installation is very, very easy on GoboLinux and you can have multiple versions of a software all running happily together without the need for every software bundling its own libraries. If some bit of software needs a specific library version, no problem! It will install that version which will still live happily with others.
How is it able to do this? Through a better directory layout, some symlink magic, and Recipes. Recipes are the means to install software; at its base, it's really just a URL with the software location and a list of prerequisites. The system will automatically go through and download/copy/compile all the needed libraries and so on before installing the main software. Recipes can install both source and precompiled binaries.
Currently, the distro is centrally-based, but there's no reason why that has to be the case. It's quite easy for individual developers to make their own Recipes for software installation and publish them. There's a local recipe store just for that, in fact.
It's also very easy to write a Recipe. In fact, there's a tool that does it mostly for you (albeit command line). Most of the time you only need to know the URL of the software you're interested in.
The main downside of the distro is that it's basically developed by two guys at present, the official release is positively ancient (you'll need to set aside a day or so to download and upgrade Recipes), and that, if things go wrong, it can take a little technical knowledge to fix (never try compiling glibc -- always go precompiled with that one). I've also, personally, never had the automatic kernel upgrade work correctly; it always requires manually moving the kernel modules to the proper place in the directory tree. Irritating, but fairly straightforward once you understand the way the new directory tree works.
GoboLinux really is the best of both worlds. Give it a try.