Does Keckleon’s ability to change type rapidly make it more valuable for scientists researching how certain types of Pokémon react to stimuli?

You would think so, but kecleon’s often capricious nature makes them rather difficult subjects. Furthermore, triggering its special ability requires constant bombardment with attacks of the target element, as this is the best way to expose kecleon to an element thoroughly enough for it to change its alignment. It’s simply more reliable to use a pokémon whose type is verified to be the target on a consistent basis. 

That and constantly attacking a pokémon repeatedly before subjecting it to further testing tends to be frowned upon by most ethics committees.