2) Why would you need to install mIRC more than once anyway? mIRC's -r command line switch allows you to have a single mIRC installation from which you can run multiple instances each using separate settings and scripts.
Well, i guess the main reason is that it is easier than creating and modifying shortcuts and so on, and of corse it is habit as this was not always possible.
Another reason may be running more than one different version of mirc.
I also think that many users apply patched to the mirc executable for one reason or another, and may not want it on all instances. (the most common being the blowfish patch)