Comment 4 for bug 553746

Yeah, you were also able to set the phone number, and tweak membership to every kind of obscure privilege, plus changing home directory. But how many people need that? If you really need a specific UID, just create the user and change UID after, that doesn't hurt, and makes most people not scared by strange settings.

Plus, not choosing an UID allows us to let adduser preferences come into play, which is very important for system-wide consistency. Note if you really need a specific profile for some users, have a look at /etc/gnome-system-tools/user-profiles.conf, where you can add MinUID and MaxUID options.