Is this worth it? I mean...does the +50 defence even make a difference? I tested it out against the guys by Firelink shrine, and there was no change in the damage they were doing me, but maybe that's because there's some sort of minimum?
Not at all. Go get hit naked, then go get hit with an armor set on, then go get hit with a fully upgraded armor and you will see. Ring of steel protection might not be the best ring to use in PVP but it can serve it purpose in other areas in the game.
i think your survivability would benefit more from the ring of favor & protection or several others.. anything that gives you more stamina to block damage with (rofp & cloranthy) or dodge it (dark wood grain) or lets you kill the enemies faster (leo, hornet, magic/miracle boosts if you use those to fight).
raising your defence seems like the least effective way to defend yourself because you have to actually get hit for it to make a difference, and the difference isn't big enough.
It is a decent ring for NG with some medium/heavy level armor where Wolf Ring's extra poise is excessive. Say, you are wearing Havel's set, you really don't need any more poise with it, but the extra defence has a slight effect. So yeah, it is worth using, but don't expect a dramatic damage decrease.