Estimation is a good skill to have. It can enable simpler problem solving and rapid decision making, and often comes up in real life in simple mathematical situations and more complex ones as well. It's also a good way to trap errors in math work; if a calculation gives an answer that's way off the mark, a student may realize it because the answer doesn't make sense, i.e. doesn't agree with an estimation that may be unconsciously made.
When we were using Singapore Math, I liked the way it would periodically (and relatively briefly) reintroduce estimation, in terms of rounding amounts in order to give an estimated calculation result. It would then ask the student to do the precise calculation and ask if the answer made sense.
All that said, I would certainly chafe at too much estimation, especially of just the counting sort. Past a certain point it can be time-wasting to sit and estimate how many marbles are in a picture.
