The only argument for tip down is speed of deployment and Sal designed it that way. I couldn't care less about speed of deployment
I carry both ways but tip down is more secure for me. If I pull out a tip up knife I have to let go of the knife somehow in order to flip it around so I can open it. Under normal draw my thumb ends up too far from the thumb hole. Under normal conditions this is not a problem but I worry that if I was pulling out the knife in a high stress situation I could drop it. I also use a tip up knife a lot while working around the house and I worry that when I need to use it on a ladder I might drop it on the ground. This is not a problem with a tip down knife so I consider this another argument in favor of tip down.
I like the increased speed of tip down although it probably never will really matter to me. It just feels better and is a more direct way of operating the knife.
As far as designed that way, that could be true. I recall that most clip knives of around 20 years ago were tip down. My theory is that there were 2 factors that got tip up started. 1. FRN handles with molded in clips didn't work well with tip down because the clip got in the way of the blade pivot. 2. Locks such as the axis lock and BBL lock didn't work well with tip down clip positions. If we had stayed with G10 liner lock knives they might all still be tip down.