I wouldn't want to contradict what Michael says, and I think this is a matter of local climatic conditions: the more extreme your climate - i.e. the hotter the summers and the colder the winters - the more it matters whether there is a floor on the hive, and the greater degree to which bees need total control over the airflow.
My evidence is based on my own experience in the relatively mild and damp climate of south west England, where summer days rarely exceed 30 C and winters bottom out around -10 C and mostly stay above -5 C. Here I find that closed floors tend to create too moist an atmosphere in winter, and bees do better with open mesh. One of my strongest hives has no floor at all, and it has just come through its third winter, which was the longest and coldest for 30+ years.
If your summer temps routinely exceed the normal brood temp of around 34 C, you may need to give the bees some help with air conditioning. Likewise, if you have very cold - and especially windy - winters, some control of floor-level ventilation may be needed, but I think you always need some floor-level ventilation.