Well lets clarify a few things. Human pranksters are a known intelligence - we know that they exist even if we can't name individuals responsible for a particular circle.
And I would certainy say that the more complex patterns are both more obviously the product of design than anything in biology - and the lack of a reasonable alternative is more solid than for anything in biology.
So in fact - especially if we include the possibility of human pranksters the inference of intelligent design has a stronger basis for crop circles than anything the ID movement has.
Now I don't think that there has been a lot of solid scientific investigation of crop circles. And that is the main reason why there has been no solid conclusion - it's just too likely to be human pranksters and that is of zero scientific interest (an unknown or poorly understood weather phenomenon on the other hand would be - so the more complex patterns are really LESS interesting to science).
In principle it is POSSIBLE some form of "ID" might become science - but the ID movement would need to seriously clean up its act and start doing a lot more work before there is any hope that they will contribute to it. It is NOT very likely that a scientific "ID" would bear much resemblance to what we see from the current movement.