Of all the calamities that can befall a hamburger, few are as subtly devastating as a convex patty. Though a bulbous burger won't taste as awful as one that's cooked or seasoned badly, it will ruin the structural integrity of the whole sandwich. A misshapen patty will make it impossible to distribute toppings evenly, to place the bun flush with the meat or to keep the meat-to-topping and meat-to-bread ratios even from bite to bite.
If you've ever encountered this problem, it probably was not in a good restaurant. That's because many restaurant chefs have figured out how to form burger patties to prevent puffiness.