Playing single player missions against the computer is a good way to develop weapon skills, and try out new strategies. While a computer opponent will not match playing against other people, it is a good way to develop skills.
If you've ever looked over someone's shoulder while they play, you can tell people who play only against a computer, versus those that play online. The single player people will know where each opponent is, and will move without a care to a certain spot, then move carefully, shoot the AI, and move on. The multiplayer people will move as if there could be an opponent at any corner, which is the case. You don't want to play single player so much that you develop the habit of only checking certain spots.