Well, the paranormal hypothesis must be the theory of last resort, that is, all conventional explanations must be unable to satisfactory explain the phenomena. Crop circles don't meet that criteria.
- Idiots with wooden boards and string have proven capable of creating complex designs.
- Designs have grown more complex with time.
- No evidence of language.
The problem is, if Aliens were doing this with advanced tech, not only should the designs be ineffably advanced, and so from the beginning, there would be some sign of an intelligent message. Like, we don't know what pictoglyphs the world over are trying to say, but we do know something is being said. Has anyone found a grammar or a syntax in crop circles? Is there any pattern that says this is a language of some kind? Any commonalities? I'll admit, I haven't investigated this line of thinking, but that should be self evident. If they are trying to tell us something, what is the evidence that we are being told anything?
Second, it really doesn't pass the smell test. Why do aliens go for all this fairy nonsense? Surely, the "universal translator" is a reality for technologically advanced species, if they have the means to travel the stars and conduct long term surveillance of Earth, why are they using lasers on farmland to communicate?