First off lets remember how an Animal dies when we shoot them with an Arrow, Hemoraging......not blunt force trauma from launching a dull object at them at 300FPS.

There will be more blood, more quickly with a razor Sharp edge, than with a dull edge, because more blood vessels and veins will be cut, and deeper penetrataion will occur. Now regardless if you get a pass through or not with a dull blade, more bleeding will happen, more quickly with a Super sharp broad head. Ethically we all have a duty to ensure that we use the sharpest blades possible for the fastest and more ethical kill possible.

Quote from
Bowhunter's Guide to Accurate ShootingBy Lon E. Lauber
I'm certain most bowhunters have killed a deer or 2 with a less than Optimally sharp broadhead and gotten the job done. In spite of that Succes, there are several reasons for having acute blade-sharpness standards. Sharp edges reduce friction and increase penetration. Super-sharp blades are more likely to slice arteries and veins upon the slightest contact. Rubbery blood vessels roll or "give" without being sliced by dull blades. A finely honed blade makes a surgeon-like clean cut. This causes more profuse bleeding because there are no jagged edges for blood platelets to clot on.

Furthermore, clean and sharp cuts cause less initial trauma to the body's natural repair system, Thus, more profuse blood loss occurs quickly, before the animal's clotting mechanisms kick in to stem the bloow flow. Conversly, if you happen to make a non-lethal hit, a super-sharp blade wound will bleed profusely at first, but heal more quickly than a jagged one.