Waste of points, better to just spend that on HP instead of rdef. Nimble backliner with 100hp can easily facetank 14+ hits from goblins, and 6-7 from even the heaviest hitters like arbalesters. And they'll always have decent chance to miss, especially if you use cover, or hide behind your frontliners, so realistically enemy ranged units would need to do absolutely nothing but focus your one ranged bro for multiple turns, to actually kill him. As long as you're using any nimble frontline, leveling rdef on backliners will just make your nimble fronts magnet for arrows, which you definitely don't want since they'll be also eating melee hits, things like goblin poison fuck them much more, and it's incomparably harder to retreat them if the fall to low hp, than ranged bros. And even if you run only BF frontline, when you're facing arbalesters or other crossbow using enemies, you want them to focus your backline even more, since this shit has a lot of armor pierce and will actually kill BF bros twice as fast as nimbles.
Overall, it's best to completely ignore ranged def on anyone, and pump more useful stats. Nimble frontliners will naturally have a little rdef from dodge, so with backline with no rdef hiden behind frontline, while enemy will try to target backliners, damage will spread out due to cover, and if unlucky archer eats multiple arrows and falls low, you just make him retreat far behind, beyond the enemy range. I don't think I ever leveled rdef, after my first couple of noobie runs 3+ years ago.