In the US (and all regulatory domains AFAIK) it is illegal to use an antenna / AP combination that has not been certified. With MIMO technology and differences in radio designs, testing the antenna / AP combinations to each allowable limit ensures that you have a compliant deployment. To your other question, the WS-AO-DX13025 was originally designed for outdoor service deployments...more specifically for stadium deployments. Traditionally, high-gain, tight service angle antennas are designed for point-to-point (backhaul) connections. We designed the WS-AO-DX13025 antenna with characteristics of backhaul antennas, but with a quick vertical drop for service. This allows for high-density designs where each antenna/AP combination can provide service to a specific section while minimizing co-channel interference with adjacent sections. We use the same type of design in indoor arenas, convention centers, lecture halls, etc but 13db gain created too much reflection in indoor spaces so we settled on a lower gain design to reduce the need of adding attenuators to the antenna indoor.
Hope this helps!
Will