not sure if that would work Tom , what happens if some people dislike each other,?
not everyone does use the thankyou option if you look aound it tends to be the same people mostly who want to thank posts and if they've a dislike for someone then helpful post or not it's unlikely they'll thank them (sounds petty but thats human nature)

personally i dont see the need for them and the post count more often than not is simply another hack that everyone installs it doesnt have to be there or switched on, but its one of those things that because most sites have it its an automatic feature provided just like,avatars, online visibility,user titles,smilies etc

it can help to be used for gauging when there's private forums that need a certain amount of posts before you can join, although (with vbulletin anyway) it automatically shows in admin regardless of whether its active on the boards or not.

but when it comes right down to it all it is really is a few numbers under the avvy in the info box.