Personally, I used Virion throughout my Hard playthrough and I didn't think he was useless. By the time other units could promote to bow using class, they'd be limited to bronze bows, which would make their attacks fairly weak even against Pegasus Knights, making them unable to one shot them. Having a trained archer since the start was a really good addition to my team.
If you're talking about grinding and such, then, really, it makes no difference. Even a maxed out bow knight can break the story chapters here. Sniper is in a disadvantage though, I guess, due to lack of counter attacks.
Considering how many things in FE6 had obvious counterparts in FE1/3, I guess they were going for something like the Archer/Hunter duality from those games. The one playable hunter had better movement and promoted to a horse class, while archers had more limited movement and couldn't promote. However, that was balanced by the one hunter and their pre-promoted versions having fairly bad growths compared to the playable archers. In FE6, they apparently forgot to balance their stats to make up for that (or did they? Its been a while since I've played FE6, I can't remember how the units compare).
