Super Telephoto versus Zoom lenses

Hi All,

Looking around for a Canon zoom lens I have been somewhat tempted to look at the Super Telephoto lenses as well, - however, I have a couple of questions (some of them can be considered "noob questions", so I hope you forgive my ignorancy):

• Both the Canon Super Telephoto lenses (L versions) and the Canon Zoom lenses (L versions) seem to be very high quality and nice glass all the way. So what is the idea behind having a 300/400/500mm lens which works as a prime lens rather than having the flexibility of the zoom lens?
• The Super Tele lenses cannot zoom, but are in general App 2,8 which can only be matched by the 70-200mm L zoom lens. Therefore I see the point in the Super Tele lenses in terms of more light and a faster lens,  - but are there any other advantages over a regular zoom lens?
• The price difference is considerable - what makes a Super Telephoto lens cost x4 of a zoom "L" lens?

Hope for a bit of clarity.

Thanks