As long as my team doesn't int too hard I can go back to shop, sell all my items and buy nothing but movement speed and make it to the enemy fountain before enemy finishes. They won the battle but you won the race.
Move speed, along with range, is a binary stat. If you are faster you can always catch and never be caught. In a vacuum nothing can be done about it. It also is constant in that everything uses move speed. Dodging, going to lane, jungling and roaming. AD doesn't do anything when you aren't using it.