Most of us have our system configured with packages from different sources. Some of you might not even be using Debian in your main system, and those who do, might not be using SID. Even though new packages can be developed and compiled in any Debian system or derivative, if you have the intention of getting them into Debian sometime you'll have to develop them under SID. This does not mean you must use SID in your main system, there are other alternatives.
Of course, you might want to install a whole Debian system in a virtual computer. That will probably be the best option for those using a different operative system in their computers than Linux (whether it is BSD, OS X, or any others). If you're already using Debian or Ubuntu, or even if you're using some other Linux distribution, you might consider working inside a chroot environment.