Well, you present an interesting point. Personally, I have planned side missions for side characters and some of them are tied to certain character arcs so the player will see more of them, that way I get to give them more depth and the opportunity for the player to know them better. Also, like any writer any opportunity to show off the rest of the characters it’s an opportunity I’ll take 
Another advantage I see to this approach is that I get to include some worldbuilding bits in their side scenes that are not necessary but do flesh out the world.
Not every character you meet has to be a RO, but if you like the idea then I don’t see why you shouldn’t do it. Maybe make it clear that side romances don’t have as much impact or content as main ROs? That way the player wouldn’t feel like they were “lied to” in a sense.
For example, like @BourbonDingo mentioned, in the Persona series you have “main” romances and “side” romances. The main romances are your close partners (Akihiko, Chie, Ann, Yukiko…), the side romances are other characters you meet during the story (the doctor in P5, Theo in P3P). The player understands that characters that they don’t get to hang out with much won’t make as much of an appearance if they romance them (the doctor won’t go to the school. Theo can’t leave the velvet room without permission if I recall correctly), and because you can’t get to the confession until you’re pretty late in the game you feel like the relationship truly developed well. P5 managed side romances very well, in my opinion, so maybe you could check that out if you haven’t already?
Also, side characters get a lot of development through the social links. In a similar sense, Fire Emblem gives more depth to characters through supports between non player characters (well, it depends on the game and the characters involved, some are… questionable)