What was the reasoning behind making it so you have to push a button to find the range of a target using rangefinders. It just seems like you tried to fix something that wasn't broken and the old system was perfectly fine in my opinion.
One and probably the primary reason for this change has been authenticity. Voted tickets even on the old feedback tracker asked for manual lasing and cool-down time.
Together with the addition of tank fire control-systems it made even more sense to make rangefinders operate manually. Otherwise we'd face a discrepancy between having a precise range - anywhere you look, all the time - with a rangefinder and having to press a key in a million-dollar tank cockpit (or less but still expensive RCWS or other advanced systems).
It's also an unification - with the updated Apex or Arma 3 key presets a single key - "T" - gives you a targeting action for the "object" under your cursor in multiple situations
- anywhere - reveal/report target to your group
+ Rangefinder - measure distance
+ RCWS - measure distance and adjust zeroing gun accordingly
+ tank and APC cannons - measure distance, speed, adjust zeroing and lead
+ vehicles or weapons with sensors - mark target