Justin Bieber Needs to Finish Purpose Tour, Says Sugar Ray's Mark McGrath

Mark McGrath's dropping some good ole showbiz wisdom on Justin Bieber when it comes to his now canceled world tour -- THE SHOW MUST GO ON!

Sugar Ray's frontman was at Catch Monday night and was blunt about his take on Justin pulling the plug in the homestretch of his Purpose tour. Yes, it sounds a little like your dad's ... "When I was your age" speeches, but Mark's speaking from experience, and on behalf of fans.

TMZ broke the story ... Justin canceled his 14 remaining shows with little explanation, other than needing some R&R.

Bottom line ... Mark says that's NOT a good enough excuse.