I also have not seen this on my tumblr blog. I've only stumbled across one or two bot-like blogs.
Also, soemtimes I'll see a like of an ancient post with no rhyme to why. I'll agree though that a good percentage of likes / reblogs are spam / bots. They also often have ads on the blog so whoever is running it will get credit when you visit the blog to see who just liked / followed / reblogged you.