For balls pitching outside the leg stump especially from a right arm bowler its difficult to guage the spin and bounce on whether it would flick the bails, so the benefit of doubt was always in favour of the batsman.
After they introduced the rule in the 90's, lbw decison in the favour of the bowler if the batsman does not offer a shot and tries to pad deliberately, now the balance is more even. These days batsman should go for a shot even if the ball is pitched outside the legstump.