Having clearance over the top tube is not important at all. How often as a cyclist do you even think of standing over it?
Pretty much very time I stop. I don't really use a rolling dismount. We all have our own techniques. I like to be able to restart my ride easily, but can't track stand, so for me its stand over the crossbar or stand next to the bike. Stand over is useful.
The suggestion that it matters if you somehow slip off the saddle just seems silly to me. In those circumstances, you aren't going to be getting feet on the ground quickly enough to prevent hitting the tube with your sensitive bits, even if not using clips.
I did not crash, I had to brake really hard, ended up standing painfully astride the crossbar after stepping forward off the pedals due to momentum. No clips on the bike. A pheasant had run out from the bank/verge as I was going down a hill (fortunately I was descending fairly slowly as there was a bend I could see at the bottom of the slope). I did not fancy killing said bird or for that matter picking mangled (possibly still living) bits of bird out of spokes or chainwheel. Only time I have actually skidded on a bike from braking so hard. Fortunately the bike held its line rather than twisting out from under me, so the bike and I both stopped upright and the pheasant, unimpressed at my own survival, safely continued its maniac travel across the country lane.
I could not go around the bird as the lane was narrow and the stupid thing was running back and forth once it saw the bike comming, undecided which verge was the safest. I think pheasants are the thickest animal I have ever come across, but they are very beautiful and the poor thing was in such a panic. Alway look back on this scene with both amusement and relief - the whole thing must have looked like some kind of cartoon to those not involved.
