You don't need them, but with the wheel off and no bolts going through them, the rotor will rotate freely around the hub. The result is that this rotation sometimes makes you spend an extra bit of time lining the bolt holes up between the hub, rotor, and wheel.
Was hoping that I would have to drill these bolts out when replacing the rotors. Sadly that is not the case. My question is how essential are these bolts really? Can I get by with out these bolts?
You are correct. That is the only purpose they serve. Often the entire suspension corner is sub assembled at a different location and shipped to the plant. The rotor banging around during the shipping is bad.I've heard the main purpose for these retaining screws is to keep the discs in place as the car moves on the assembly line. Seems reasonable.
This is the only effective way to remove these that I've found. Pretty much standard on all cars...impact driver works great for those.
File:ImpactDriverWithBits.png - Wikipedia, the free encyclopedia