How would you do LL on D? You'd have a hard time looking at the cube unless you constantly kept looking at it like it were between D and F. The algorithms too, might be a problem, but not as much as not being able to see what you're doing.
There's another topic that discusses cross on top, as doing the x2 after the cross may give you a quick full inspection that subconsciously speeds up your F2L, even though it's physically slower.
x2 is a pretty fast move for me, try it yourself. Solve the cross on top, and do x2 and go straight into the first pair nonstop using x2 for inspection? I can't do it, but maybe I could with practice.
I just did two averages of 12, one with cross on top, on with cross on bottom.
Cross on top: 24.09, (25.97), (16.71), 20.05, 18.50, 25.65, 23.78, 25.36, 19.36, 17.66, 19.80, 19.90. Average: 21.42
That was actually better than I was expecting, especially after the first two solves. I did get faster after a little while, but it was really tough, and I found myself doing something like D2 instead of U2 a couple different times.

The colours messed me up as well, because the green and blue (or red and orange) are switched when you hold it upside down. I found myself pausing more as well and messing up F2L more, especially on the first pair, because I was concentrating on not messing up the cross.
Edit: Also, non of these solves had a smooth transition from cross to F2L.
Cross on bottom: 17.53, 18.09, (22.22), 17.05, 19.44, 17.22, 19.33, 20.88, 19.68, 19.77, (14.15), 17.38. Average: 18.64
Not much to say about this. Very nice average, almost 3 seconds better than the cross on top one. Less F2L mess-ups and a much smoother F2L. I'm guessing my cross times were faster, as well. My times were also more consistent, with only two solves slower than 20 seconds. I guess a lot of it can be attributed to familiarity, because I had a couple fast solves with the cross on top (all solves were non-lucky).