It seems that in the diving community there is an ongoing debate about the necessity of wearing a snorkel. Some people believe that a snorkel is an essential piece of dive gear, others see it as an extra piece of gear that is there for emergencies only. Still others complain that it is a hazardous extra that causes water resistance and entanglement issues. So what is the correct answer? 

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.

The suggestion of always doing your deepest dive first has been regarded as much more of a rule of diving for decades rather than just a suggestion, but is it really that important? In order to answer the question, we need to first figure out where the guideline came from.
The suggestion first seems to have appeared in diving literature in the early 1970's. It was referenced as a guideline to follow in order to reduce or eliminate the need for decompression stops. For example, diving first to 100ft for 20 minutes then to 40ft for 60 minutes with a surface interval of 1 hour you do not need to complete any decompression stops, but if you dive in reverse, starting with the 40 ft dive followed by the 100 ft dive, you need 2 decompression stops for a total of 26 minutes. 
The reference never provides any hard evidence that not following the rule will be detrimental to your health. 
​The AAUS (American Academy of Underwater Sciences) decided to look into this matter further to see if there was sufficient evidence to create an actual diving rule. They looked at current practices and diving risks associated with diving in reverse. The conclusion was that as long as a diver is being safe and working to avoid decompression dives and the injuries associated with decompression sickness there is no need to follow the deepest dive first guideline as though it were a rule. 
This doesnt mean toss out everything you know, it simply means, if you find yourself doing a deep dive after you have done a shallow one, more than likely, nothing will happen to you as long as you stay within your limits.