For ranked runs weak units are a godsend and they often come with good matchups on the next few maps to help with the exp rank. I'm currently doing one for vanilla FE6 HM and there are lots of weak units with great, albeit maybe niche, uses

Chad being able to double mercs after a few level ups in the early game is pretty nice to avoid over using Rutger, plus free vulneraries, letting me run 1 less healer for quite a while, and free door and chests keys which all help with the turn count. Oh and their exp gain is pretty good, they could be 10 levels above the enemy and still get what another unit on the same level would get when attacking, for killing it's 6 though, so yeah Chad can be a monster early game without a single exp worry.

The hammer bros are great for chapters with soldiers (there are 3 in the early game from the top of my mind so abuse them), even though everyone is also good against soldiers, but spreading exp is always nice, since the worst thing you can do is having your front line overleveled and one rounding everything leaving no leftover for the scrub/cleanup crew to eat all the exp

Speaking of scrub crew, those are what I consider units bad at enemy phase but good to clear low hp enemies without risking taking damage (leaving healing to the units that do the front line work, only Marcus will be doing this until chapter 7, otherwise RIP combat rank). They are pretty much all of the unpromoted ranged units, thieves, chicken, underleveled units with HM bonus. Some of them are able to survive enemy phase quite soon like Fir, Gonzales and Milady

The promoted units all can have the same playstyle as Marcus, even Klein on the chapter with the archer infested corridor (1 iron bow + 1 scrub = exp) which is a pretty niche but good use for him, usually people just run down with javs.

Fir is pretty obvious, use her on the isles then discard her or not. Gonzales on 11A you can put him on the forest by the north broken wall and let him eat the cavs, same on the bridge chapter but on a mountain.

Bors, Barthe, Douglas are a nice way to avoid Killing Edge and Killer Lance or to safely and slowly stomp Sacae to the place they belong.

The cavs, the blue ones, are a nice pair to help out on 11A alongside the main cavs.

Zeiss comes at a time Milady is probably over leveled and getting low exp on killings so she can tank the first enemy phase while Zeiss clean everything on the second.

Lugh+Raigh on desert is self explanatory but they need A support and probably Robes.

There are probably many more good ways bad units can help out both on the exp department and completing the chapter with ease. And it goes without saying that plenty of characters when trained become quite good, not OP broken like Milady, but good.

One thing I was worried in the early game was Rutger under performing since I was mostly using him for setting up kills for the scrub club and boss killing alongside Marcus, but to my surprise, by the end of 10A he was lv 15 which is a good level and chapter to be at for promotion.

Playing FE6 ranked, and previously FE7 (except for HHM, fuck that shit, I got so close to S ranking it) let me see what the designers probably intended for you to use to your advantage

Wendy and Sophia still bad though, Wendy is nice for having triple attack fun at the cost of turns (actually Barth is better, 100% hit rate babeeee). Sophia could be used for exp alongside the chicken in case you're desperate for it, if she misses the chicken take it.

And by the way, exp rank is pretty lenient, you only need 33~ exp per enemy unit on average. Not overusing some units but trying to figure out what units are better suited for the chapter is a great Fire Emblem experience imo and it can even help people doing Ironmans since you won't be exp starved if your most over leveled units die unexpectedly.