We have developed several stores for different clients. Some specialized in flowers and gifts like to have this option for it simplifies the shopping process. In this type of stores the shopper buys for someone else so the delivery is going to be to a third party. If you requiere registration, the shopping process becomes budersome. On the other hand our clients recon that when they have the data of the shopper they can promote recurrent sales.
In my store I use anonymous checkout and also I've modified the shopping cart page so that when you click "checkout" it goes directly to the billing address, instead of going to the loging/anonymous checkout page. This reduces one step in the checkout process. I also skip the shipping method selection page if there's only one shipping method available. Both are relatively simple changes.