The divers who advocate taking the snorkel out of your mouth are the ones who do deep, very long breathhold dives. For this kind of diving, there is a definate reason to not use a snorkel.
But for the diving I do, which is fairly shallow (20-25 feet), and in rivers, I use the snorkel all the time. I do not push the limits, and establish a pattern for submerging and coming up. It starts out quite short for the dives, then gets longer as my body adjusts to the cycle of breathhold diving and surface intervals. I don't hyperventilate, and take no more than three deep breaths before diving. When the urge to breathe comes, I surface, stay there for at least a minute, then go down again until that urge to breathe comes. As the cycle goes toward 15 minutes, I find the dive times extend naturally, without hyperventilation.
This cycle of diving and adjustment period is important in the prevention of Shallow Water Blackout (SWB) mentioned above. I encourage you to enjoy snorkeling and breathhold diving, but also to learn the mechanics of SWB, as it can be quite devistating and happens to the best of the breathhold divers. There are several discussions on it here, so look it up, study the mechanisms, and make sure that you don't get caught in that situation.
SeaRat
PS--RCohn had this link, which is a very good reference, on shallow water blackout:
http://www.freedive.net/chapters/SWB3.html
For personal info, see this description of when I almost did not make it back:
http://www.scubaboard.com/showthread.php?t=124511&highlight=Ratliff
Finally, Wikepedia has a very good description of the problem, as does the diving medicine page:
http://en.wikipedia.org/wiki/Shallow_water_blackout
http://scuba-doc.com/latenthypoxia.html