I tend to only like comics as a sort of way to "save" them. Subscribing is my way of showing I like the series, likes are my way of giving feedback on the comics/pages I thought were above-and-beyond and therefore want to be easily able to find in my likes.
I don't think likes are ever bad, though. At least, on Tapastic. On a site like Tumblr, I would say likes don't really benefit the creator because a reblog does so much more for exposure and letting the artist's work be seen.