And why do men have nipples? Useless.
This argument is based on the assumption that an Intelligent Designer should not make imperfect things.
Why do you assume that?
I assume it because Intelligent Design was always primarily about getting religion - i.e. Christianity - into public schools, especially in the US.
It started with explicitly Christian creationism; that got ruled unconstitutional. So they stripped it down and called it "creation science"; that got ruled unconstitutional, too. So they stripped creation science down and called it "Intelligent Design"; this got ruled unconstitutional, too.
The whole thing has always been about getting something that's as Christian as possible into public school science curricula, but not so overtly religious that it's declared a church-state violation.
The "intelligent designer" was always the Christian God; that was the whole point. It's also why the ID crowd never talks about "intelligent designers", plural. If they were being honest, they would recognize that there's nothing in the public face of ID that would necessarily require all "intelligent design" to be by the same designer... but they don't, because the ID movement is nothing more than an effort to insert Christianity into public schools and hypothesizing multiple intelligent designers would run counter to this purpose.