Sometimes their bowling became less effective (IIRC Yovich lost a yard of pace, Matthew Hart got the yips) so to maintain a spot in the playing XI they needed to bat in the top 6.... Glanced back at some old scorecards, all-rounders like K Mills, M Orchard, J Yovich, N McCullum, Devcich, M Hart often batted at 6 or even higher. Some of this is genuine improvement of bowlers' batting skills, but some of it was surely robbing younger batters of a chance, to fit in more bowlers for what reason? ...
For others I think (Mills, NMc) it was more about having 5 bowling options, and they were good enough to bat #6 in domestic FC.