Well would you agree the tail helping out the batting order happens more than a fifth bowler bowling teams out? If nothing else, having a tail that can hang around rather than be walking wickets is very important to stretch the innings.
I have a few responses.
You said a couple posts back that you've seen lack of a tail cost teams victory, that can be said of basically every team that failed to reach a target, but in reality it's the batsmen whom would have failed. It's the guys paid to bat from 1 to 6 that holds that responsibility. Yes it's very useful for a couple to hang around and stay with the top order guys and to help build a total, but you don't need an all rounder for that.
When Ambrose was routing England for 46 or any of the great bowling spells for that matter, I highly doubt that your no 8 was going to be the stop gap to halt the flood.
The role of the fifth bowler is less abstract. The effectiveness is in the action alone, any results are just the icing. I think Kallis played and handled his role perfectly. Gave the guys a rest, broke the odd partnership and was sterling to Steyn and co in the slips. From that perspective he was way more important than a few extra runs at the end of an innings.
With Gilchrist I can say, yeah that dude won games, and that was due to the circumstances under which he scored his runs and especially the speed (PEWS), but for the most part, have seen that with a lot of the bowling all rounders tbh.
But it's cool that we see things differently. Despite my efforts no one is adding cordon work to these comparisons despite the fact that I feel that it's at least as important and depending on the bowlers and situations, much more so.
With Bradman, Sobers and Gilly do I need more batting? No, but with Marshall, McGrath and Steyn do I want to ensure we take all possible chances, hell yes.
In a weaker team would I prefer the bowling all rounder, likely, but again it's not going to win us any games and just staunches the bleeding and gets us a few draws, or extends a few games.
But my thankless crusade goes on.