Renting a video camera is good way to get what you need for the time you need it.  Purchasing a camera may not always be a good idea, especially if you need the camera for specific period of time, and you never really ever use a camera besides that.  The difference in cost between renting and purchasing a camera can be enough of a factor to make rental seem much more affordable; in addition, renting makes a wider assortment of cameras available to you that you may otherwise not be able to obtain due to their cost.  Renting a video camera is a common practice for film projects.  Owing a camera is just not practical for certain things.  Putting a bunch of money into buying a high speed video camera may mean that you cannot afford to buy or even rent an underwater camera in the case you should need one.