From an electrical engineering perspective assume we have 3 points of entry to the 1522, power, data and antenna. Each of these can transmit a lightning surge to downstream devices ie a switch or an upstream device the ap.
Generally the antenna will be the most vulnerable point of the installation to lightning strikes. If there is no surge suppression between the antenna and the ap then the ap can be damaged, also it can potentially transmit that surge down the data cabling, assuming the power cabling is protected as you say it should protect in both directions on the power side, but not always.
This probably means it can manage surges into the ap from the power side from a strike to transmission equipment and the transient overvoltages that may be caused.
Worst cast scenario your antenna gets a direct hit, the ap fails, the voltage is transmitted down the data cabling and takes out the 6500 its directly connected to! Nobodies going to say you done a great job and were just unlucky and you wont be winning any popularity contests
Its a question of risk, with an inline arrestor on the antenna cable you are protecting your ap etc. If its on a building and not the highest point its unlikely to get a direct hit. If its on a lighting column and the antenna is the highest point its possible. How much lightning do you get?
As an ATP partner the first thing Ill tell you is that an inline antenna arrestor is a bad idea on a couple of points. 1st they don't really protect the AP and secondly, they reduce the signal strength by causing at least 1db of attenuation and in most cases 3db (50% of the signal strength). Lightning arrestors for the data side and power sides will protect downstream devices but the AP itself is really only protected by luck. A direct hit and a 1500 series AP is toast. I have deployed nearly a 1000 of these and this has always been the case. There are a few enclsures that help with this. They generally have a metal case and the antenna are mounted low on the enclosures (not a good solution for 1520 series) and the enclosure has a lightning rod on the top. THe attempt is to catch the lightning and re route it before it hits the actual AP antennae. Not very successful I would think and not worth the extra 700.00 per AP.
I also have a similar requirement to provide lightning protection for 1522. Here the antennas are installed on top of the building and AP installed inside the building (antenna connected with antenna cables to the AP1522). But as per the 1522 deployment guide, the ethernet port of the 1522 is protected by Gas Discharge tube (GDT) lightning protection. In this case , is it required to use another ethernet surge protection ?
It is best practice in any outdoor to indoor deployment. Most time I will see lightning protection on the antenna cable one outdoor and one indoor. If the unit is outdoor, then usually a lightning protection on the Ethernet outside and indoor. Again... No matted if the unit says there is protection. Better safe than sorry.