I'm far from an expert, but I'll share some of my views on using motion blur.
I think almost all action shots need some amount of blur (either in the subject, the background, or both), as long there is a sharp area for the eye to lock-on to. Otherwise, the photo can take on an appearance of being staged; like an athlete posing, a racecar parked on a track, or a model airplane hanging from an invisible string:
Untitled by
Colin Whittaker, on Flickr
I was kicking myself after this one because I loved the lighting and dramatic clouds, but forgot to drop the shutter speed enough to get the props blurred.
As Maximilian pointed out already, the OP's first shot of the baseball player has enough blur to show the action. Similarly, a wee bit of blur in the wheels/tyres of a car makes it look just right:
Untitled by
Colin Whittaker, on Flickr
Apart from just conveying a sense of motion, panning is also a great way of isolating the subject and removing distractions in the background. In the OP's frozen shot of the motorcycle, my eye is drawn away from the main subject by the people on the left side and by the trees/sky in the background. I find this is especially applicable when the subject is small in the frame:
Untitled by
Colin Whittaker, on Flickr
Here, I didn't quite get the centre car tack-sharp, as I would have liked, but I still find that is draws the eye immediately despite being relatively small.
In cases where you have a thin-enough DoF, this becomes less important. The photo of the baseball player is a good example - the background is out of focus anyway, so no need to blur it further with panning.