While most training agencies have standards dictating that students will have snorkels during training, I tend not to use one in my everyday diving. There are situations that no snorkel should ever be used on the mask: cave/cavern, wreck penetration, ice (hey, hard to snorkel back to the hole), and SAR teams generally don't use them. I know that I have been criticized for not having a snorkel from my regular buddy, but personal preference reins king in this instance. However, a good j-style snorkel is an excellent resource for rescues (mouth to snorkel rescue breathing).