In a nutshell it´s about giving your opponent more opportunities to make a mistake in the meaning of sklansky´s fundamental theorem of poker.

The narrower (is that a word?) your range is at any given given point in a hand, the easier it is for your opponent to take the correct action in response.

If you want to merge or to polarize your range is primarily not depending on your opponents abilities. It´s the nature of the specific action you take and what your opponents most likely response will be. Imagine a villain who folds 80% of his buttons to a 3bet and another who folds only 20%. Vs whom would you rather merge your 3balling range from the blinds?