But a better reason in-universe is that you're supposed to be commanding an army, not picking statblocks from a list. You might only bring a third of the party to deal with the Evil Overlord, but the other two-thirds still need to be ready to hold off his Army of Darkness.
You can't both be not giving people experience and be having the rest of your army's abilities matter. The deployment limit issue is mostly a gameplay thing since characters, in-story, don't necessarily "level up" so the guys who aren't fighting are probably still doing just fine.
Meanwhile, your top fighters are not tireless, and having them in the thick of battle all day every day is just asking for trouble. Just because only one game has S Drinks doesn't mean no one in the others feels fatigue!
The series almost always handles this off-screen anyway. FE4 has long gaps in between most chapters, and most chapters are implied to last quite a while, which likely includes down time happening. GBA games have support conversations often mentioning all kinds of off-duty stuff that suggests that being deployed on every map doesn't mean you aren't resting, and a lot of games have chapters that even have it directly pointed out in dialogue that everyone is going to be resting up after the current battle.
All that said, it is my belief that the primary justification of the EXP Ranking is "discourages people from relying on a small, overlevelled team", and to be perfectly honest I am OK with that. I personally find strategies like "dump a hojillion BEXP on this unit, have them solo chapters" to be repugnant.
BEXP isn't in any game with rankings <_< Anyway the problem with that justification is...well...it kind of sucks. I prefer to use a specific team, being forced to deploy random scrubs I don't want to use isn't exactly fun. It's why I don't like Thracia's fatigue system all that much and only refrain from being too harsh on it because it makes staffs only quite broken instead of SUPER broken.
But those limited criteria are exactly my issue with it! Particularly since I tend to view "recruit all units and keep them alive" as an implicit goal of any non-Shadow Dragon playthrough.
And that's exactly why I like it! It's not asking me to do any weird side stuff, it's not asking me to collect awesome stuff and just let it sit in the convoy because it's considered bad strategy to want to actually use your resources, it's not asking me to use characters I don't give a **** about (the fatigue system is, but not the ranking guidelines) for the hell of it, it's asking me to beat the game efficiently, recruit everyone, which is a goal I like to go for anyway when Xavier is not involved (because **** Xavier) and to do that however I feel like. Use whoever I want, use whatever stuff I want, play the game however I enjoy it within the confines of finishing in few turns, which is something I like to do in some playthroughs anyway. And the turn limit being as low as it ends up putting the resource management levels up there with other games anyway.
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We clasped our hands, our hands in praise of a conquerors right to tyranny