How on earth do you think Wii Sports Tennis use motions more effectively? It only has two different swings... forehand and backhand.
I haven't played Wii Sports Tennis in a while so I had to look it up. It has more than just forehand and backhand shots:
Forehand Topsin: Twist the Remote from the buttons facing up to the buttons
facing down.
Forehand Slice: Hold Remote with the face of it towards the sensor, followed by
swinging upwards and twist the remote away from the sensor.
Lob: Swing from a lower area to a higher area.
Backhand Topsin: Twist the Remote from the buttons facing down to the buttons
facing up.
Backhand Slice: Hold Remote with the face of it away the sensor, followed by
swinging upwards and twist the Remote towards the sensor.
The only shot it is missing is the flat shot, although I'm not sure whether or not how far you twist the Wiimote as you swing affects the amount of spin on it. I don't remember having a problem with it registering different shots than what I intended, but I'd need to play the game again to see how it works.
Since all Mario Power Tennis does is replace buttons with motions, it seems like it would have been easy to just set the Wiimote to recognize five distinct motions as opposed to making them as subtle as they currently are.
Besides, this is two years after Wii Sports so Nintendo should know how to get the controls to work in a tennis game.
Proper detection of swing direction would have made it worth the wait if nothing else. Swinging later or earlier is (and always was) a dumb way of choosing directions.
I think it worked fine in Wii Sports Tennis because the game was designed with that in mind. For a game designed with a traditional controller in mind, it isn't going to work very well.