Hi Tay,
My wife got herself a nice MacBook Air so I went looking for a hardshell case to protect it and keep it looking new. Here are my observations.
First of all, I doubt very much the cooling is an issue. The thing is, the manufacturers of these cases pay attention to the air vents and engineer their cases so they don't interfere with cooling. The exhaust vent on the back side of the MacBook Air is completely unobstructed. Since MacBooks don't rely on radiant heat from the metal case, having a somewhat thin layer of plastic over the top of the metal isn't going to affect anything.
Neither the MacBook Air nor the Pro have intake air vents, so there's nothing about the case to be concerned with in that regard. All in all, airflow really isn't a problem with hardshell cases.
As far as the lid; I'm skeptical of claims that the extra weight is going to be a factor there. On my wife's, the shell that covers the display is a fraction of the weight of the metal shell plus the display and digitizer glass. It really doesn't weigh enough to put any significant extra strain on the hinges.
All in all, I would highly recommend getting a hardshell case. It'll provide an extra layer of protection against drops or bangs and will also have the benefit of keeping your MacBook looking pristine. They weigh almost nothing and won't interfere with cooling or usage.
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