There are two basic types of stock photography licenses. Rights Protected (RP) and Royalty Free (RF). Your choice between the two depends a lot on the needs of your current project.
Rights Protected images are licensed for a specific usage and duration. The usage/s determine the cost of the license. RP images tend to be much more exclusive and have the benefit of a detailed past usage history. Therefore you can go forward with the confidence that a competitor has not recently used the same image. For an additional fee an RP image can be protected so that it cannot be used by another in your industry for a set period of time.
Royalty Free images are licensed at a set price that is usally determined by the size of the file needed. They can be used again and again without paying an additional fee. While the types of projects RF imagery can be used for is almost unlimited, certain restrictions do apply (see agency's licensing agreement when purchasing). With RF there is no licensing history available and the same images tend to appear more often in all types of print and web usages.
In our eyes the decision is quite simple. If you or your client would be embarrassed to see an image you had chosen for an important project being used by a competitor, Rights Protected imagery is the way to go. Reputation and brand identity are far too important to take a chance on using an image with no usage history as a major focus of any project. On the other hand, if you need an image for a smaller usage and/or the thought of seeing the same image being used by a competitor is not a concern, then Royalty Free would work great! By applying these definitions on a project by project basis it should become quickly apparent which type of licensing is required.