Calculation is important for understanding math. It should not be the only thing taught, but it should definitely be in the curriculum, and in a substantial amount, at least in the first few years. I think it lays a lot of ground work for further mathematical reasoning. Probably the most compelling reason, though, is that in mathematics you're assumed to have a certain degree of competency with computation. If you can't add or multiply small numbers in your head, you would be out of place among the current crop of mathematicians. While not everyone is trying to be a mathematician, it seems that it's not a good idea to remove something so fundamental that is essentially taken for granted in certain fields.