api, reservist rank is mostly useful when you have a lot of offliners in a battle. with the way most rankers handle the process these days (which i hate, personally), that only those who ask for a rank will get one in order to give priority to onliners, when you rank reservists it sorta makes recruits the equivalent to privates, so reservist would single out the players with a lower chance to get picked by the system to fill up the side (say low level, low hp players who will also be offline). it was a lot more useful when ranking was handled properly, and we didn't have battles with more captains than privates, and more recruits than privates.
before the invasion of tanks and supertanks or special guns, battles were more like 1-3 generals, and maybe 5-8 captains, and the rest were privates. in a properly ranked battle, i would find the option to rank someone reservist to be useful if you have members of the owning town signed up to fight, your side is overfull and you want to make sure someone more fitted (higher hp, better gun, better gear, more experienced or whatever) would take those places. there have been cases when the system would leave out players with a private rank and let in members of the owning town that were just recruits, when a lot more players were signed up to fight and at the fort. ranking those townies reservists was a quick and safe way to make sure all those who got a private rank would actually keep their places.
these days is just meh, because either you don't see full battles on most worlds, or the battles are mostly 10-25% generals, 40-50% captains, 5% privates and whatever is left are recruits. i wonder what would have happened in the alamo if it was fought that way...