Hmm.. this is a long thread, my 2c is probably worth nothing by now, but I might as well say it.
I love thanks messages. I made an image specifically for subscribers because I want them to know that I really, truly appreciate the their readership and I am very grateful that they are giving me a reason to continue working. I know people don't like copy pasting. I try to make it personable by including their name and if they have something interesting in their profile, maybe I incorporate a little blurb about their description or comment on their avatar. But the bottom line is, no matter how I am posting the message, I am thankful for that person's readership.
I also like them because they're a way for me to find other comics. I would say 30-40% of the comics in my list are there because I have seen other people post their thanks messages on other's walls. I would have never found some of the comics I truly enjoy if I did not see them on other people's walls - why should promoting be a bad thing?
When those same people post their thanks on MY walls, it just means that anyone else who visits my wall also has the opportunity to enjoy that same comic that I like, and help get that author that I like more readership too. I like to help out other creators, and I think they deserve fans, so having a showcase of thanks messages on my wall of comics that I like is a way to help fellow creators get more readers. That's why I post mine in public rather than private, because I want my comic to be seen too!
Finally, I like them because sometimes if the person I'm thanking comments back, I start a small conversation with them and make a new friend. If someone posts on my wall, I try to say what I like about the comic so they can keep doing the good thing that they're doing.
I will be considerate if I see a message in someone's profile that says "don't post thanks messages" or something like that, then I will just move on and not post anything. And if someone doesn't have such a statement, or doesn't like them, then they can just delete them, simply, it doesn't matter to me. All I am doing is conveying my gratitude for liking my story.
I understand people do not like them cluttering their creator message walls, that is a legitimate problem I agree. I thought Tapastic had a separate tab for creator messages only with the new update, but I guess they do not. All messages that I post have pictures in them, so if anyone needs to see old messages they can just go into the image tab to see stuff, so it doesn't affect me much.
TL;DR: I like thanks messages because I like letting readers know that I'm grateful for them, it helps me find other comics, I like to help promote other creators and it sometimes gets me new friends. It would be nice to have a tab for creator-only messages.