Think of registering for copyright as an insurance policy.
If you don't register, you still own the copyright. However, if infringed, you most likely will be footing the bill for the legal fees, pretty much be limited to collecting actual damages only, etc.
If you do register, you now have additional benefits avail, such as the courts ability to order the defendant to pay your legal costs, statutory damages up to $150K per image infringed, as well as the actual damages, etc.
Most infringements that are not registered go no where since no one in their right mind wants to spend $18K to collect a $2,542.43 infringement. No attorneys are likey to step up and take it on a contingency basis, so you will be asked for a few grand up front just to get started, and regular payments up front as the case progresses. The party that infringed is more likely to blow you off knowing that the legal costs of bringing a lawsuit in most cases surpass what could be collected, and most photographers are not likely to take that path.
Infringements that are registered are more likely to be taken on a contingency basis if the infringement is substantial. Also, the infringing party is more likely to step up and want to make a deal knowing that if they loose they could be ordered to pay a significant amount over and above the actual damages. They many also be aware that photographers and attorneys are more likely to file suit when the copyright is registered vs not because of that.
For $35 for a CD full of images (1, 500, 10K, whatever you can fit), its a great insurance policy should an infringement occur.