Microwave has broad applicability, high reliability and availability, relatively good ease of use, and relatively low cost. You do need a license to operate most microwave systems, and the more popular frequencies are congested and difficult to get licensed, especially in the major cities.
Like infrared, microwave requires line of sight, which is problematic within major cities. If you are lucky enough to get a frequency licensed, all your work could be undone because someone builds a building between the two points on your microwave link. This occurs more often than you might think.
Microwave enjoyed most of its popularity in the 1980s as a "bypass" alternative to go around the local telephone companies when long distance got cheap but the phone links to connect to the long distance providers got expensive. The logical response of enterprise users was to dump the local telephone company and use microwave to connect directly with long distance carriers of the time such as MCI and Sprint.
While the financial motive was the primary driver, it took the enterprise user only until the next cable cut to realize that microwave also had use as a disaster recovery technology. Microwave provided the ultimate diverse route because one cannot dig up air.
In the 20 years since its use as a "bypass" technology, the feature richness and reliability of microwave has increased, and the cost of these systems has dropped significantly. For example, on many systems there is a greater choice of interfaces with Ethernet and T1 interfaces being commonplace.
But as stated earlier, you have to secure an FCC license to operate a microwave system. The manufacturer can help you do this, and there are also numerous consultants who can literally be found in the Yellow Pages to help with the same issue. If you are looking for true diversity at reasonable cost and at higher reliability than infrared, microwave may be the ticket.