This could lead to security issues. .com is a file extension for DOS executables. Someone could have created a malicious .com file called "google.com" that somehow ended up on your hard drive, and when you do:

/run google.com, mIRC would execute the file, and the malicious executable google.com would run.

It can also conflict with another .com filename that exists on your hard drive.

Ex:

Let's say you want to visit www.format.com , using you're way of doing:

/run format.com

would cause the 'format.com' program to run.