Of course, checking how often Mac users use the right mouse button would really skew things, would it not? Besides, popularity and usefulness are not the same thing. If a two men were standing on a street corner handing out money and one was handing out free $20s while the other was handing out free $10s, which would be more popular? Probably the one giving out $20s. Does that mean the $10 bill isn't useful?
The reality is that there are some very good use cases for the caps-lock (as others have pointed out). If it isn't hurting anything where it is, then why move it? What other key would you put there that you would want to hit with your pinky to do something else, that most people would find as a useful improvement? Right now if I accidentally hit the caps lock, I get capital letters -- a nuisance but not terrible. What if you replace it with the ctrl or alt key as some have suggested? There could be far worse ramifications.
Regardless, popularity does not dictate usefulness. Chances are, you will never have to use the flotation device on an airplane or the oxygen max. Based on frequency of use, they must not be very popular. However, for the right user case, they are very useful.