Bots usually have a greater following, which makes them very useful, especially when they also resteem your content. Your content gets spread much further.

Bots that upvote usually bring you in more money than you spend, although it depends on the bot. The @minnowbooster is a good one for example. You'd have to test that and check their voting power. Upvotes with a high voting power (and high reputation) bring your article to the top of a tag list, which also means that much more users find your content.

Bottom line: Bots look like cheating, but all they is make the system better;-)