During the majority of dives they are completely worthless. If worn attached to the mask, they create a significant risk of entanglement, they can inadvertently unseat your mask, and they can interfere with the immediate use of other people's equipment. They interfere with gas sharing, but do so at the risk of dislodging your mask when donating gas in an emergency.
As for entanglement, the snorkel as typically worn is attached to the mask strap, and left free to rotate when not in use. This creates an entrapment hazard around the divers head, since the upper end of the snorkel tube can direct monofilament, kelp, or debris into a position where it can either exert force to dislodge the mask, or come loose from its parent object and hang from the snorkel, creating a hazard in the manipulation of other equipment.
In addition, if you are diving with a long hose for gas donation, the snorkel is in a position to catch the hose as the regulator is deployed, interfering with immediate donation, and pulling off your mask in the middle of a potential emergency.
On the surface, the snorkel neither conserves gas in your cylinders, or does not help you breathe better. All it does is create additional dead gas space which detracts from your capacity to get full breaths of fresh air. The snorkel does not provide you with air - the air is already there, and all you have to do to get at it is breathe. Exhaustion is made much worse by the increased CO2 resulting from the additional dead gas space. Within the context of a proper gas management plan, sufficient gas should be available to continue to breathe from the regulators for the duration of any anticipated surface swimming if conditions require it. If not, or if conditions change during a dive rendering the remaining gas useless for surface use, the diver is best served by breathing the surface air without the problems caused by use of a snorkel.
The best argument in favor of using a snorkel is the ability to breath without inhaling water when conditions are rough, as one might do without the snorkel. This is not true. In any sea state, the clear air is in the bottom of the wave troughs, and you need to time your inhalations to click with that position. Spray may be present in extremely heavy seas, but this originates from the wave crests. In the troughs, the airspeed is too low to support a liquid water load. Accordingly, it makes sense to draw air from the lowest possible position, instead of extending the height of the air inlet by using a snorkel. In addition, spray and breaking waves and so on can be managed simply by breathing through clenched teeth with the tongue as a splash shield. Even if you have to resort to doing this, you are better off this way than with the snorkel, since in a panic reaction you will breathe instinctively, which may be will not help due to the position of a snorkel inlet in any particular instance.
Swimming, or accomplishing any useful work in a heavy sea state is necessarily going to represent an elevated workload, and raise heart and breathing rates. This is the last place you want to introduce additional dead gas space as well as moving the air inlet to a position that does not enable you to discriminate against incoming water from breaking waves or spray without distracting your attention from other things.
The snorkel does allow you to breathe with your face in the water, but this is generally not an effective position for any purpose. Surface swimming in dive gear is more effective on your back, since you can generate more powerful fin strokes. You can follow a compass course this way, too. Surface swimming at all is actually less efficient than swimming completely submerged due to the effects of the water and the snorkel is a trap for kelp, sea weed or other floating debris.
Awareness is reduced with your face in the water, especially in areas with poor visibility. Keeping the head out of the water while surfaced enables you to see all surface traffic, hear all surface noises, and send and receive light and sound signals appropriate to the situation. Also, if you consider the scenarios which would require extensive surface swimming like emergencies, it makes sense to present as large a visual profile as possible, to help people which could be searching for you on the water's surface.
Snorkels are appropriate for snorkelling, in SCUBA the use of a snorkel is limited, and can be replaced by simply putting the reg in your mouth which is a better solution anyway.
