All these users registered a long time ago and successfully entered their registration info the first time, but suddenly decided to format their computer and install mIRC 6.10 but took the time and wrote their reg information on a piece of paper... but failed to remember the whole Help > Register process.
Or they purchased a new computer, or the purchased a second computer, or they want to use their registration key on a PC at school, or at work, etc.
These users unscrupulously generated a NEW registration key because their old "serial" stopped working in 6.10.. and they can't enter this new information because the menu item is still missing. (i obviously dont know if this is how mIRC behaves, but it would explain why people cant find the option)
I just tested using an illegal key, it was one that said it was for 6.03, it successfully worked in 6.1, so this whole argument is simply fiction.