While installing MinGW, the users are warned about making changes to the System PATH variable:
You should not alter the system path variable in any manner, or you will cause all sorts of problems for you and your computer!
In my experience, these folks tend to know a lot about Windows, so I'm inclined to take this seriously. However, no reasoning is provided.
In the past I would usually change the System PATH variable such that my usual environment would be available for my Admin account as well as my user account, and haven't experienced any trouble. Should I change this practice?
Could anyone provide some example situations where adding to the System PATH will cause problems for the system?
you will cause all sorts of problems for you and your computer!
without giving you specific information smacks to me of someone who really doesn't know very much about Windows. If they're knowledgeable they ought to be able to give you specifics.