"The first pieces of equipment you should buy as a diver is a mask and snorkel". This is the statement that we dive instructors around the world often use.
The snorkel along with the mask and fins are considered the basic core equipment a diver needs, however after a diver qualifies from his open water course it is the first thing he wants to remove.
So let us look at the pros and cons to diving with a snorkel in place.
The snorkel is compulsory piece of equipment when teaching entry level courses with most diving agencies and this helps to make the student learn how he can save valuable air while doing a long surface swim to the dive site, also if someone had a low or out of air situation then the snorkel would be invaluable in poor surface conditions.
However the snorkel can be uncomfortable, get in the way, unclip and fall off (we have all seen plenty of snorkel fish lying around the reefs). Also it can pull at your mask in a strong current causing leakage. The emergence of the pocket snorkel is a definite option for experienced divers and it is also available for you to go on a snorkeling trip with the family. They are small enough to fit in your BCD and most have a quick attachment.
My opinion on snorkels is that they are a must when diving but on certain days I would assess each dive independently and make my decision before entering the water, although I am seriously thinking of purchasing a pocket snorkel