Wubi is a great way to try Ubuntu without having to re-partition or reformat. However, a user may get confused about how to "run" Ubuntu once it is installed. Assuming they figure out that they have to restart their computer to switch back and forth, the text-based boot menu is not intuitive for those who are used to doing things with the mouse. Even for experienced users, it's easy to get distracted while the computer is rebooting, miss the boot menu, and end up booting into the wrong OS.
Suport a installer like wubi for other distro of linux (Fedora, openSuse, etc).
ex: If i use Fedora, and I want to try linux for a while without making partition on the disk. It would be nice to have that and after a while to uninstall it, if I want.