I would tell you that lens choice has everything to do with what you are shooting, where you are shooting, the conditions you are shooting in and what your desired results are.
In my work I use everything from a 10.5mm all the way to a 600mm lens. It just depends on what kind of images you want to produce.
The 70-200mm lens is certainly a good all-around lens. And if you are looking to buy just one lens, a zoom is a good choice as it will give you the benefits of several lenses in one.
I am not sure if I helped you, but I would be happy to answer any questions you may still have and try to point you in the right direction for what you want to do.