Basically what >>4317
says, but consider this: THC "burns" too: at high temperatures (temperatures reached by combustion but not by vaporization) THC degrades to non psychoactive substances.
Here's why people who prefer smoking do so: vaporizers, providing less heat overall than flame, do not heat up as much herb at once. While it's easy to smoke down a 1 gram bowl in a bong, with vaporizing it really isn't possible to "instantly vaporize" your herb, for the same reason as burning food taking an instant and cooking it taking time. (I will admit my (at this point obvious) bias towards vaping, but I am a longtime smoker and continue to do so on occasion.)
There are ways around this: one is the obvious route, powering up your vaporizer. Many desktop vaporizers are quite strong. The E-Nano can achieve 1 hit complete vaporization (though 2 is far more common) but it does so kind of by cheating: it only vapes a tiny amount at a time. However, it does vape it thoroughly and apparently from very small amounts can provide a lot of vapor. Another solution would be a third lung, as in the Volcano. From what everyone says about the Volcano, this solution is apparently pretty good. It helps that the Volcano's heat and fan provide ideal heat distribution over the herb, cooking it perfectly.
Also for what it's worth >>4317>whereas a weed vape will usually have a metal screen to spread out the heat.
Vaporizers such as the E-Nano or Volcano actually use convection heating: they run hot air over your herb to heat it. Many vaporizers, especially cheaper portables like the MFLB or even some nice ones like the Pax 2, use conduction heating, which does use a heating element, mostly commonly stainless steel.