That type of thing is fine, within reason. From the
License Info FAQ:
Q: Can I install on more than one PC?
A: During the two-week period after you have purchased or restored, you can use the Install Key you received to activate the program on your other PC’s. You may use the program on your work PC and on your home PC's with a single license.
Basically, if you have a few computers that you own and you use, you are welcome to use MC on all of those machines. The FAQ answer makes it seem like you have to do it within two weeks, but that's not quite the whole story. Once you buy MC, it generates the "Install Key" file and sends it to you. This file "lasts" for two weeks, but after it expires, you can generate another one with the "restore" process (which you can do up to 10 times per rolling year).
It is a bit on the honor system (though that two week limit and ten restore limit is designed to prevent rampant abuse). The way I, personally, interpret this is that I use one copy of MC for my home machines (my collection is constantly shifting, but I have three active boxes for "home use" right now). I also use MC for work at the office, but I felt this was a different enough type and purpose of use (and a completely separate set of machines that I don't "own" myself) that I bought a second license for the office.
This licensing term is a big part of the reason I was originally willing to click the "buy" button for the first time way back in the day.