That's a legitimate question. The gold investment and the numerical gains are in a very poor ratio for the last couple of levels. It can still be the difference maker though. For example you might need 3 instead of 2 hits if you lack just that little bit of attack. Similarly, you might not be able to outtank an enemy if you lack a little bit of def/hp (especially if you also did the same for your healers; it will add up). So I think you should just bite the bullet and properly max your units. It's true that ultimately, the goal is to replace your silver units with higher rarity units, but unless you're a whale or get lucky with the gacha it can take you quite a while to do so. For example, Leeanne will stay by your side for a very long time if you don't have Bernice, Gellius or Deine. Even if you do get one of them, you might need a second HA on some maps. Do you really want a unit like that to be gimped, even if ever so slightly, just to save some gold?