The reason to use more than one is simple. backtrack to practice and learn hacking (white hat), ubunutu or another easy distro so i can learn to use linux before spending most time in back track, and windows 7 for games.
I would use BT on a 4
+GB thumb drive. Do things the easy way first. I could tie my shoes with my hands. I could use chopsticks too to make it more interesting. lol
....learn hacking (white hat)...
A simplified definition of white hat is a person who was hired to find, break and plug security holes within legal boundaries. Knowing the law can be a must for professionals in this field. Grey hats are a cross between both bad and good. The software he or she is hacking/cracking is often times not owned and is copyright restricted. That makes their actions illegal. They might choose to do several good things after finding vulnerabilities and or problems. Black hats are the ones whose actions are purely illegal. These individuals likely never disclose their activities or what they do. Script kiddies and hacktivists are polar opposite. That is quick rundown of them. Unless you have legal rights to the software you are hacking/cracking or are hired by a lawful company, you are
not a white hat. This was not meant as a flame bait. Relax.

....ubunutu... ....back track... ....windows 7 for games.
This is perfectly OK. Ubuntu is nice as an everyday use OS. Windows is great for gamers plus many professionals needing a desktop environment. Back Track is much more a package of tools than anything else. Whatever you choose to do, make sure you know your rights before diving in. Neither Maximum PC nor its members are responsible for your actions.