This topic comes up on VIN a lot, especially with data indicating that early spays/neuters delay closure of growth plates. Anecdotaly, a lot of large breed dog owners feel that pediatric spays/neuters cause the animal to have a more gangly appearance as adults, and prefer to wait until their dogs have had a chance to mature a little. In a shelter situation, I don't think there's any argument that fixing animals at 8 weeks is beneficial both in making pets more adoptable and in helping to curb the overpopulation problem.
Personally, I think manditory spaying and neutering is going to be hard to enforce and will affect only responsible pet owners anyway. If someone is running a puppy mill, they aren't going to be willing to pay an "intact pet fee", and will just stay under the radar. Responsible breeders don't make any money off their dogs in general, and having to pay an additional fee to be allowed to breed will certainly squeeze them financially. Most communities already charge breeders a higher fee to license their animals than they charge for a spayed/neutered animal... why do we need even more on top of that?
Someone on VIN also made the point that in communities where manditory sterilization has been enforced, it actually increases euthanasia rates and decreases the number of people licensing their pets because they don't want to have to spay/neuter. I think this is a case like breed-specific legislation... Sounds good to ban the breeds that have the highest bite rates, but is there any evidence that breed bans actually work?