In specific usecases (consider an index on timestamp taken at the moment of document creation) DB designer may have intrinsic knowledge on what the index page density should be and can take the responsibility for balancing conflicting goals of index compactness and page split avoidance.

Mongod is already capable of implementing two different page split strategies: 50% and 90%. So we can just provide means for the user to specify a custom percentage when creating a new index.