First lets look at the Pro's of wearing a snorkel as a diver:
It can save air during the surface swim to and from the dive site or boat
If you run out of air you can still swim to your dive site or to shore face down while still being able to breathe
You can breathe through it while waiting to board the boat during off shore dives, especially in rough seas. No one wants to get a mouthful of ocean water while waiting to get out.
Some con's to wearing a snorkel as a diver are:
The breathing resistance is greater, even during a surface swim, than simply breathing through a regulator.
The amount of air used during surface swimming and while waiting in line is not enough to justify switching mouthpieces out on the water.
The bulk of some of the higher end snorkels can cause drag and get in the way of the mask straps and other hoses
If you have an alternative air integrated source the mouthpieces are usually on the same side which can cause confusion.
During technical dives, such as cave and wreck diving, and even during some faster moving drift diving the snorkel can become a hazard as far as entanglement and increasing the risk of it getting caught and pulling your mask off of your face.
So what is better boils down to what kind of diving you will be doing and what your personal preference is. For many divers, it is just another piece of equipment that is there "just in case of emergency", for others it is more of a hindrance than a helper. Some things to consider are whether or not you are comfortable using a snorkel, how much surface swimming you are going to be doing, and if you really need all the features of some of the newer snorkels or if you can get by with just a bare basics snorkel. If you find yourself in the situation where you see valid points both for and against snorkels, you might consider having a collapsible or fold-able snorkel that can be put in a BC pocket and used only when it is truly needed rather than constantly be available attached to your mask.