While I agree that clothing is unimportant, I think there is a greater problem. In many Muslim countries, these so-called unimportant customs such as attire are actually very important. In the more moderate countries, certain attire is frowned upon, and in the more strict ones, it's illegal.
So what happens when you combine two cultures-- one that is open to other cultures and wants to let foreigners enjoy their values, with one who's very culture values not allowing such freedoms? I think the answer is obvious: one culture will dominate the other. At the very least there will be tension, and at the very worst there will be violence. What's more is that this example is only a superficial one. We still haven't explored deeper cultural touchstones that are rooted in religion.
This has all the ingredients for disaster. We've seen it time and time again. Whether it be Irish Catholics with Irish Protestants, or Anglos with Francs, cultural conflict is inevitable. Muslim culture is antithetical to Western culture, they simply value opposite things. Where I am from, there was great conflict which revolved mostly around language. How much more conflict will there be when language is the least of concerns?
Now, I'm not saying that we should kick out anyone, or restrict anyone from coming in. At the same time, much like if I go to Saudi Arabia and am told not to wear a tank top or shorts, I would quickly capitulate. When the shoe is on the other foot, they refuse to. So it's not that the clothing itself is the issue, but it's a sign of a greater problem that lies beneath the surface.
Here is a quote I pulled from a Qatar campaign aimed at foreigners:
I'd say that's very reasonable. If I don't agree with their customs, I simply won't go there. Culture is important, all of them, and just as I have no interest in imposing mine on anyone else, I would like other people to respect mine, too.