Really nice comparison. I don't have something equal to post but I'd like to add my opinion:
Example 1, motor bike:
Here i definitely prefer the panning pic.
It has the right amount of blur and is always preferable when shooting from the side and when you want to show the speed and the dynamics of racing. I would prefer frozen action here when it comes to front view.
Example 2, baseball:
You already have motion blur in the "frozen" pic when you look at the ball.
The blurred pic is already too artsy for me because of the long movement of the arm and the strangely big and white left leg.
I suppose motion blur would have worked perfectly if you had made it a little bit more than the "frozen" and much less than "blurred" pic.
IMO it would have been perfect, if you could have caught the last part of the arm swinging and a bit more of the ball flying. The backswing behind the head is too much.
Generally theory says use panning and/or motion blur if nothing else is showing the dynamics.
If you have motocross with a lot of mud spraying or skiing with snow or water drops/waves at surfing or water ski this will work equally well and it is up to the personal preference which one is better.
I'm of the opinion that if you are going to pan with the subject then the background conveys the motion and the subject should be as sharp as possible, if you are wanting the subject to convey the motion then don't pan, the background should be still and retain definition. Take the bikes and swap each to the other picture.
I like the amount of motion blur in the sharp pitcher shot, the second is way to artsy for my taste, unless it is supposed to be artsy and not about motion blur and conveying movement, then I'm the wrong person to comment as I really don't get impressionist (is that right) art.
I'm afraid panning at a different speed from the target just looks like an epic fail to me, but then what do I know, I'm doing my other hobby not photography for a living!
I did try something "artsy", cars racing at Goodwood, first car panned and as sharp as I can get it, the second is both out of focus and motion blurred to convey the chase. Not sure it really worked! SE0A7481_DxO by Graham Stretch, on Flickr
And in the dark. SE0A7531_DxO by Graham Stretch, on Flickr
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:
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:
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.
Since it is about opinion, this is mine.
But I will start with someone else's; it was said on this forum (by someone else): "how could you stand out from the crowd if you do what everybody is doing?".
My opinion, pan is a photography technique. Convey motion is only one of them. Unfortunately, everybody (almost) thinks that is its sole purpose.
OK, what if you use it in very low light where your ISO and noise would be too much for you to accept because you need to freeze the motion? A friend of mine asked me to shoot his sister during speed skating competition once (because I have a better gear) because he was not able to get decent photos the year before.He was shooting at ISO 12800, 1/500sec, I did it at 1/25th sec, ISO 800. Guess how surprised he was.
It can be used as well to separate someone from others, when their speed is not the same; like in bike race, or the opposite - when you want to show that they have the same speed?
It can really be used for different things, the more you use it the more these scenarios show up on your path.
I don't have two photos that showcase the difference as you did but here is an example of my panning. GPC Montreal Criterium National-0003 by Alain, on Flickr