In 7th ed, core choices were 3+ with a minimum of 3.
In 8th ed, core is 25% + of your army
In 7th ed, it only took 5 models in the flank to disrupt and remove rank bonus
In 8th ed, it takes 10 models remaining after combat to disrupt and remove rank bonus.
In 7th ed, 2000 pts was the norm
In 8th ed, 3000 pts is more the norm (more sales with rules changes)
Therefore detachments SHOULD COUNT TOWARD THE 25% CORE REQUIREMENT. This would return the detachment benefit and allow more points to special and rare options