Dear me. I'm surprised that the thread title is even a question. Maybe in the 1880s when cyclists heroically performed gymnastics to get aboard an Ordinary or Penny Farthing, and had to dismount and do it again every time they had to stop, the new fangled Safety Bicycle was seen by diehards as an option for softies. But if you have the option to have a bike which lets you match your frame size to your leg length, why not do it properly, and give yourself some ballroom?
Perhaps there is some competition based macho motivation to have the tallest frame practicable, which outweighs the painful possibility of whacking your gooseberries on an unyielding top tube when you stop? Based on my experiences as a long term short legged cyclist, when the smallest decent bike I could find in my price range when I started out was 20" with a horizontal top tube, it's definitely do-able but gives no leeway for unexpected stops on uneven surfaces, bad cambers or off road. This, with the expectation of sliding forward from the saddle and straddling the top tube for planned stops, and getting into the habit of just having one foot down when stopping. I did thousands of miles on that bike, and became very fond of it. I still regret selling it, due to its history.
When I later bought a mountain bike, which by its nature had a lower frame, and found I could stand over it with both feet on the ground, and airspace between the top tube and my undercrackers, it was very liberating. In the intervening years I've had a few bikes, but I've always made sure I had decent space between me and the top tube. Maybe modern small frames with 700C wheels and a fair bit of seat post showing aren't as aesthetically pleasing as a traditional taller frame with horizontal top tube with shorter seat post, but since I'm not looking at it when I'm riding it, it's the practical aspects that matter.
As I've got older and less flexible, even this set up is not so easy to mount and dismount from. It's a truism that for older riders, it's not the riding a bike that gets difficult, it's the getting on and off. I've had a few folders, and the ease of getting on and off those has to be experienced to be believed. Another aspect of getting older is that everything starts heading south, and your dangly bits get danglier, so you have to make allowances for that, too. It's quite painful to kajagoogoos when getting on and off a bike with a top tube. If people like to put their feet down while sitting in the saddle, well if it works for them, fine, even at the expense of losing pedalling efficiency, but it's not for me.
Since I discovered recumbents the issue of top tube clearance has faded into the background, and I don't use my diamond frame bike a lot now.
Leg length still matters, though. Sitting on a recumbent bike last year that I'd hoped to buy I found myself on tippy toes. My short legs still impose limitations even on a bike without a top tube.
