Once the Nation Battles are introduced she will have her uses Wutan. Until you get 20 girls for each nation, they still have their uses, even if they are obsolete when compared to the other ones, or are later completely swapped out for other non/event girls (still good half year from now on at the this pace at least).
No matter how bad a 5*, its still better than 2/3/4* most of the time, and as such, having the girl maxed out Skill/Item slot wise, will be usefull when the need for her arises.
I like how in Kampany Girls, you could use those girls that were weak/swapped for better ones, for crafting, material gathering, etc etc. Gave you at least some reason to keep them around at decent lvls, since it affected their % chance of getting the task done properly.
And if nothing else - do it for the completion sake. Having a character, that could have been maxed out with little effort, just lay there being completely worthless because "she was not good enough to bother, thus I had her become completely garbage by not even maxing out her skill/slots" is very wasteful, especially towards girls that you would like to max out from dupes, but they wont and wont drop.
Oh and also let me quote nazrin "
Waifu Lives Matter"
(I like that very much, thank you
)