The extra stats and surviability of pairup greatly increases your options of how to tank Berserkers plus with more creative build or lucky levels (as I have been considering just base kits and average stats to keep things fair) the use of support abilities like inspiration, supportive, demoiselle/gentilholm, lily poise, rallies, etc. you can push plenty of units into survivability range. That being said tanking a round alone has one huge advantage, you can then have a unit adjacent to let whomever was frozen pair with, and escape the range of the freeze staff.
That sounds good in a vacuum, but when that room also has two maids with freeze staves and seven generals, in addition to Hans, who I tend to Entrap and kill with ranged weapons from the northern room (this causes two more Generals and Berserkers to spawn from the stairs. Luckily, the reinforcements with ranged weapons will try to attack from through walls unlike the units that are in the room to begin with), it sounds more like robbing a police station. Luring is also one thing Xander's good at, since he outruns them. That said, while retreating, I may try to have him kill the Tomahawk users on counters. Axebreaker is a level 15 skill, and with the AI being what it is, that just might cause them to attack someone else, which might not be what you wanted. Also, I'm fine with using pair up when I feel I need to do it, which is mostly to avoid enemy dual strikes or to help retreat, but I'm not going to field a unit that's only good for being a pair up bot.
Which are rare, even in Conquest.
I fail to see it - Mozu starts at level 1, and requires a Heart Seal that could've been used by a better unit, which means opportunity cost rears its ugly head. Setsuna doesn't measure up either - she has good speed, and not much else. She also joins later than he does in Revelation (chapter 16), which doesn't help when she's already worse off than he is. Nor does it that the only weapons stronger than the Fujin Yumi are silvers, which drop your strength and skill after combat, in addition to having a crit evade penalty, and the Pursuer, which requires S rank and drops your defenses, in addition to coming late.
EDIT: Also, I'm not seeing everyone else gain little experience by that point unless you are playing Revelation and try to recruit her immediately after chapter 7 (personally, I find it best to wait until chapter 8 before playing her paralogue). And I still fail to see why you think growths are the be-all and end-all, because it's not like growths can be counted on to salvage a bad unit.