Without the aid of panning, all the elements of a song pile up on top of each other. This forces you to deal with all the frequency masking issues.
Everybody loves taking shortcuts, especially creative-minded people. This means that we are naturally looking for quick solutions to fix our problems. Yes, tracks-yet-to-be-mixed are a problem. Once you finish a mix, you solve a problem.
"...if you can make cheap stuff sound good, you'll make the posh stuff sound great." - Fraser Smith