the reason it's fun is because it's a super lazy, still enjoyable version of ping pong. To make it seem like a marketing pitch: "it's all the thrills of ping pong without the effort!*" (*not including the $360 + tax you need to spend to buy a wii, the game, and another controller + motion plus)
against the computer it reaches a point where each match is really easy, but against a friend, you can go back and forth for a good while. I don't know if it's as intense as Ping Pong on the 360, since that is a better game even without the motion controls, but like I said before, I would totally buy a dedicated hardcore ping pong game for the Wii now.
btw, dogfighting and a lot of the other multiplayer only games are just as boring as the regular versions. Though, I think the plane flying game has some appeal in single player since it has crackdown-like orb hunting.