I spent some time late last week messing around with Tiger Woods PGA Tour 10 on the Wii to see what MotionPlus brings to the experience. Quite a lot, it turns out.
As you can see in this video, the biggest addition to the game is the ability for it to detect your wrists position and translate that into the angle you are holding the club. What that means is that if you twist your wrist or body during a swing you'll end up with unintentional fade or bank shots.
It didn't appear that the actual swing detection was any more precise than it is with just the remote, but that makes sense.
Playing through several rounds of golf both ways, I can say without hesitation that MotionPlus adds a whole new facet to the experience, making it much more precise and a bit more dependent on player skill.