The problem with not setting a max number of sub pages is that some theme authors will then try to add 12 pages, and the reviewers will have nothing to back them up when they ask the author to remove them.

The problem with setting a max number is that everyone will use the max number...

The team has tried to be flexible in the past and review settings on a case by case basis.
There are always exceptions to the requirements (except licensing).

It is highly unlikely that anyone is going to mark a theme as not approved because the author asks if they can use 4 sub pages, as long as the content follows the other requirements. That is not what the team is trying to accomplish.

-The problem we (reviewers) are seeing is that when we ask theme authors to motivate why they need something extra, or when we encourage them to explain why they need to break specific requirements: The theme author never replies.
It is a communication issue.
Instead of motivating why and request exceptions, they remove the settings without adding as much as a comment.

-And in the long run this means that requirements are not updated to reflect what the authors really want.