I know that Ubuntu is based on Debian, and I also heard some people talking about leaving Ubuntu and migrating to Debian. Why would they do this? Are there any resonable arguments for doing so?
Ubuntu is based on Debian. It's Debian plus other stuff.
It's based on the same layout of system files and package manager. Ubuntu also has a slightly more liberal policy on packages it will consider for acceptance in the distribution - Debian is rather strict that included software obeys free software guidelines - Ubuntu is not so strict, and therefore has utlities that install closed-source drivers and patented media codecs.
Ubuntu adds more repositories, therefore more packages, and has more software available for it. The desktop environments also feature Ubuntu themes. Lately Ubuntu has been adding some unique features, such as "Ubuntu One" (a "sync" service, seems to be like an integrated Dropbox) and the "Ubuntu Software Center."
Debian also has a habit of introducing releases slowly. So you will often get older versions of software with Debian, but they are well tested and stable. This is good for a server environment which Debian excels at. Ubuntu is more bleeding edge, with more chance of encountering bugs.
Ubuntu also has a very friendly graphical installer.
Generally if you are familiar with one your skills will transfer to the other. Ubuntu seems to me to work with a greater variety of hardware out-of-the-box (with typically difficult hardware under Linux such as video cards and wireless adapters). Debian is good if you want to start with a bare system and only have specific stable and well-tested software you need on the machine - again, really good for a server environment.