Post 1. People may be thanking several helpers for their efforts, in which case PMing them all may not be practical. They may also not know about the PM feature.
Post 2. I think it's a nice gesture and gives a much nicer feel to the board. When you're helping in quite a number of threads it's not always practical to PM everyone.
Here's what I think about this:
Everybody is thankful if you help him, why do they need to say "Thank you"...
I know that if I help somebody he will say "thank you" in his head or maybe in his prayers.
I don't expect from anybody to post it here... but also, I don't mind...
But, again, that's my opinion.