Guards are pretty whatever for the normal content. There's probably no situation where you absolutely need to have one (vanguards can do as well usually), and if not, then Melantha is perfectly sufficient.
This is untrue considering how people always joke about how Arknights is basically guardknights. Once you get higher rarity guards, they're basically the most OP class in the game. Once you have your 5/6 star guards leveled, you can pretty much have a team full of guards and clear most of the content. Examples of OP guards being Silverash, Hellagur, Blaze, Lappland, Specter, etc etc.
I think it's less guards in general and more duelist guards (ie the ones that have high HP, high attack and block 1 enemy) that are considered a bit meh as far as late-game, hard content is concerned. It doesn't help that there are DP-on-kill vanguards can often do their job as assassins just as well. That said, it's not like they're terrible or useless or anything, just not as "meta"; they're still definitely useable in most levels.
Also, I finally found that one Exusiai x Mostima picture I was looking for earlier.