I have seen that criticism of many sports cars over the years, but always by people that are missing the point. Sports cars aren't for hauling crap, they are for delighting people who travel in them. Any creature comforts or luggage space a given sports car may have is a bonus!Some would also classify the Sky as "silly" given its lack of utility.
I grant you that GM did a notably poor job of offering luggage space compared to many other sports cars, particularly on the convertibles (the coupe is quite a bit better) but space wasn't their prime mandate when designing the cars - fun was. I think they nailed that part of it.
Some sports cars have next to zero space for anything - Lotus 7, Bugeye Sprite - but both make you laugh out loud when driving them and that fulfills their purpose.