Kicking is such an important part of football that you need to have two players on the active roster to handle those duties, because you'll really be up **** creek if you only have one and he gets injured. The Eagles lost both their punter and kicker in the same game a year or two ago, and they had to resort to their long snapper, Mike Bartrum, doing the kickoffs. It was atrocious.
So, if you have two, it then makes sense to have each specialize in one of the two kicking disciplines, and back each other up in the other. Punting and kicking are different enough to warrant that kind of focus.
In kicking, you're hitting the ball with your instep, and driving the ball with both your legs with motion across the body. Because of that cross motion, its really easy to pull the ball off target, and kickers have to practice their mechanics to be accurate.
In punting, you're hitting the ball with the outside of your foot, and your power comes from body motion forward and driving upwards with your legs. Because you're hitting the football with the outside of your foot from a position high off the ground, its really easy to completely shank a punt.
You know how baseball pitching mechanics will mess up a quarterback's throw and vice versa? (see, Chad Hutchinson and Drew Henson) Punting and kicking are different enough to expect to see a difference in performance between multitaskers and specialists.