The primary advantage of Backtrack is that someone else did all the work a recent broad-selection of tools into a single distribution, and it makes them all available by default without you having to install them or search for them.
You could certainly acquire the source and compile/backport/install everything present on the Backtrack disk onto any generic distribution.
BackTrack as opposed to, say, Debian?
There are several tools included in Backtrack that are not included in the Debian repositories, but many of the tools are available. Debian excludes some packages because it has a pretty strict philosophy of Free Software.
Debian, and some other general purpose distributions have a slow release/update cycle. When it comes to security testing, you frequently need to stay extremely up-to-date. New exploits are being found, meaning you need newly released tools to detect them.
Are the repositories actually different?
Recent versions of Backtrack are based off Ubuntu, but there are many tweaks and changes to packages to make them more useful to a security professional.
Backtrack Linux FAQ: Why can't I just add the Backtrack repositories to my Ubuntu install or the Ubuntu repositories to my Backtrack install?
We highly recommend against this action because Backtrack tools are
built with many custom features, libraries and kernel. We have no way
of knowing how they will perform on a non Backtrack distribution, plus
you will very quickly break your install. Also if you chose to add the
ubuntu repositories to your Backtrack install, you will most certainly
break your entire Backtrack install very quickly.
What's more, it seems like people who try BackTrack complain that they can't get the most basic things to work 'out of the box'.
It isn't designed to be a new-user distribution. It is designed for security professionals. These are the type of people who you would expect to have done some research to make sure they have hardware that is going to be directly supported under Linux without having to jump through an of the more annoying hoops (ndiswrapper/firmware downloads) to get the equipment running.
If you are planning on doing anything with WiFi then selecting the right card is extremely important. There is a relatively small percentage of WiFi interfaces that will provide all functionality you might want.