I strongly suspect that those antennas are highly optimised and they lend themselves to being mounted in optimal locations. A couple of rubber duck antennas will work in the same room, but keeping that stuff up and away from all the other gear will pay dividends on the fringes of your wifi coverage.
I agree entirely. Look at these lovely radiation patterns:
https://help.ui.com/hc/en-us/articles/115005212927-UniFi-Network-AP-Antenna-Radiation-Patterns
I strongly suspect that those antennas are highly optimised and they lend themselves to being mounted in optimal locations. A couple of rubber duck antennas will work in the same room, but keeping that stuff up and away from all the other gear will pay dividends on the fringes of your wifi coverage.