I think it depends upon how many times you post. If I'm not on much I can't rep, then when I am on regularly, I can rep more often.
It's also a personal issue in a way since sometimes we appreciate what someone has posted in a private way, it's a personalised way of appreciating a person's contribution on the forum and for sure you have all given some great advice to me.. I was well prepared for my surgery and post op., thanks to you all...
If I leave a rep, it's a private thank you between the person and me, based on the good work they have done for others and on the forum.
I also think the 'Thank You' facility gives the writer a lift to know they are appreciated and I often use this to thank people.
I'm really thanking you all for your confidentiality, support and love xxx