None of my hives have a bottom entrance. There is no particular reason for one except that it's the way it's always been done.

That said, a bottom entrance works fine as long as you keep it free of weeds in the summer, don't have skunks, keep it free of dead bees and snow in the winter and remember to put the mouse guards on in time...
http://www.bushfarms.com/beeslazy.htm#topentrance
How to make a top entrance:
http://www.bushfarms.com/beestopentrance.htm
Historic reference to top entrances:
"I had a neighbor who used the common box hive; he had a two inch hole in the top which he left open all winter; the hives setting on top of hemlock stumps without any protection, summer or winter, except something to keep the rain out and snow from beating into the top of the hive. he plastered up tight all around the bottom of the hive for winter. his bees wintered well, and would every season swarm from two to three weeks earlier than mine; scarcely any of them would come out on the snow until the weather was warm enough for them to get back into the hive.
"Since then I have observed that whenever I have found a swarm in the woods where the hollow was below the entrance, the comb was always bright and clean, and the bees were always in the best condition; no dead bees in the bottom of the log; and on the contrary when I have found a tree where the entrance was below the hollow, there was always more or less mouldy comb, dead bees &c.
"Again if you see a box hive with a crack in it from top to bottom large enough to put your fingers in, the bees are all right in nine cases out of ten. The conclusion I have come to is this, that with upward ventilation without any current of air from the bottom of the hive, your bees will winter well without any cobs."--Elishia Gallup, The American Bee Journal 1867, Volume 3, Number 8 pg 153