I think it is important for a priesthood holder to consider how a mission fits into their priesthood duty, and for members of the church to encourage every young man and woman who desires to serve a mission. That being said, personal revelation matters, and it is not our place to judge the reason someone does or does not go on a mission. That can lead to those who do not go (for any number of reasons) to feel out of place or unworthy. I also don't think that you can't reach the same spiritual heights if you do not serve a mission. Just look at Thomas S. Monson and Russell M. Nelson.