I use them, posting to my subscribers' walls.
I like them, because a.) it lets me show my subscribers that I do notice them subscribing, whether they are silent readers or the type who comments frequently, b.) encourages them to interact, by reaching out to them, even if it's with a similar message/image for all of them (there are only so many hours in a day, and personalising every message would take too much time). It kinda lets them know that I'm approachable, you know? I like talking to people!
To me, it's just part of the general landscape of Tapastic, I guess? I feel like they contribute to the welcoming and friendly atmosphere. They might seem impersonal, since it's pretty much identical messages etc., but the creator still has to go through the manual act of posting them all to individual subscribers' walls, which takes time and attention at least - and I do this weird thing where I have a personal rule that I have to manually type out the name of each subscriber. The rest of the message might be copy-pasted, but the names are always manually typed out, no matter how weird the spelling.
I guess it's some kind of digital.... handwriting... thing? I don't know. >.< It's not like the subscribers will ever know, but I will, and I guess that's the point.
... speaking of which, it might be time to draw a new thank-you picture, but that might have to wait till January. I'm so busy I'm slowly going cross-eyed.
(also, this thread comes as a magnificent coincidence - I've just spent an hour thanking new subscribers! XD)
ETA Re: the PM vs wall-posts thing. When I get a subscriber who has, for some reason, chosen to hide their reading lists, I usually PM them the image out of respect for them wanting to keep their reading-list private. I don't do it 100% of the time, because sometimes I forget, but I try to.