Because, why not?

A good argument for tiling window managers is the fact that many people who don't use a tooling window manager still use their WM as if it were. My wife, for instance, uses Windows for work. Every program is always maximized - in Office, even the dialog windows like the file chooser are, because they all just use the main window.

I would say that, if you're going to work this way anyway, use a WM that's designed for it, not a floating WM where you've hacked tiling onto it. Back to the example of Windows, it is a PITA to find the right window; the way Windows presents multiple windows of the same program is hideous. It's painful watching my wife hunting to find the right Excel spreadsheet, or PPT. Desktops (or tags) are far superior, and if you have a lot of things open, tabs.

The exception is graphical work. Applications like Gimp, which have a lot of toolbars and frequently changing windows, are better floating (IMO).

But, basically... use the right tool for the job, and usually a tiling WM is the right tool. Desktop widgets make for cool screen shots, but who users a computer where they ever look at the desktop?