My problem with this argument is that I do not think that unibody design with or without a shell is that great at dissipating the heat.
I disagree.
In my tests (albeit informal, only with my computer, and with my daily use) I saw no difference between having the case on or off. I posted the results somewhere in this forum a while back.
And, I think the unibody does a fine job of keeping cool. It seems to be running well-within normal parameters and my fan rarely moves beyond idle speed. A cooler computer would, of course, be nice, but cramming all of these high tech components into a tiny area and forcing the computer to perform demanding tasks is going to generate heat. There is no getting around it. As long as it is not excessive, I see no problem.
I can't speak for the case the OP is considering, but I have an Elecom one and it doesn't block any ports. Obviously, it adds weight, but it is negligible (I cannot tell the difference having it on or off).
As for dust, it definitely does collect in there, and I tend to clean it every weak or so. It is not a huge amount, but enough to bug me.
No matter how careful you are, Mayhem is out there, and this tiny investment has already protected me from damage, so I highly recommend it.