It is nice having all of them handy, for the above stated reason. I like to keep sharp on all of the differences. As long as you understand the variations from one OS to the other, Troubleshooting on the fly (without an “answer” PC there in front of you) becomes easier because you have a background.

As you had stated, it also helps with application compatibility.

The best reason of all, if you only have the one PC and an OS crashes, you can always boot to the other, get on the Internet, and figure out why the other crashed! You could even put it off for a while, and figure out what is wrong over time. And you wont be a lUser, without a PC.