If you're looking for a desktop/end-user operating system (even for a server of some sort), you're not really looking for L4. You might as well just use Qubes; the security model will be similar (in that you'll be "using" hosted Linux, not the underlying OS directly).
L4 gets interesting if you're building another platform.
Well, I think it's an interesting idea in itself to build a full operating system based on a secure kernel, even if it's server/CLI only, runs in a virtual machine as a prototype (think GNU Hurd). Isn't it?
If you're going to build a new OS, sure. That's how you should think of L4: as a toolkit for building new operating systems.