The design argument was decent enough until we started understanding things about our world or until you bring a religion into it.
Crudely put, it says that if it looks designed then it probably is.
But those that use the argument also want the designs to be imperfect too (so that they can explain the imperfections in nature). They explain that god is perfectly capable of designing imperfect stuff. (Which is true because, by definition, he can do anything.)
They go on to say that even if something doesn't look designed to us, it may be designed anyway because we can't know the mind of god. And presumably vice-versa.
In other words if you want to use the design argument plus religion, you can have it all ways.
Life, don't talk to me about life - Marvin the Paranoid Android