There is evidence that about 20% of mites fall off as they emerge, and they supposedly can’t get back in if they fall out the hive. That claim comes from Mark Goodwin and Michelle Taylor (Control of Varroa, revised and reprinted 2013). That said, they don’t make any comment of the effect this has on hive health.
A potentially better argument for the open style of floor is the additional ventilation. Where I am gets something like 1125mm of rain a year, which is quite a bit. Keeping the hive ventilated is an additional benefit.
A third reason is that the bases I’ve got are so nice to use, and cost no more than the older style wooden type.
