Teams in lower leagues are worse at everything, so you could make that same argument for anything (i.e. "having a well constructed team is less important because you don't always need a well constructed team to win"). Arguments of importance are relative to other strategy considerations so this doesn't make any sense.
If you have 9 GS when others have 7, and everything else is equal, your team will play roughly like it has double the other team's salary. This means you can be terrible at everything else and still win. To me that sounds important.