+def charms… any use?

I'm confused..
the way you phrased that makes me think you're asking "should I bother keeping solid charms where one affix is defensive, such as life, FHR, .." in which case the answer in a very obvious "yes, yes you should". 3/20/20, 5/20, ... most of the good/truly valuable charms have a defensive stat on them, if not two.

And yet, the follow-up makes me think you're actually specifically talking about charms having +defense on them which makes a lot more sense as a question as it isn't quite as obvious.

For that one, defense on charms is a prefix. As a result, you're primarily competing against dmg/AR, elemental damage, mana and resistances. For large charms, you're also looking at MF as a possible prefix. And for grand, you're additionally looking at both, MF and skills.

With that in mind, the dmg, AR and elemental dmg bits will probably beat def by a clear mile in any scenario on a weapon-based build but for casters, those are entirely irrelevant.
Resistances are obv. critical as a stat but if you're already capped anyways, it may be effectively useless on charms.
Mana is a solid addition for many a build but 0 concern for various others.

So for small charms, I could see a distinct group (albeit niche/rare) that would like def on their charms. Not so much "to get def" but rather "to get something with at least SOME theoretical value added".

Large is effectively the same thing for anyone who doesn't care about MF or even wants to actively avoid it on a base finder.

Grand however, I don't see. Sure, def is still a theoretical benefit for everyone but even if you don't need any of the other stuff, every build out there will get some sort of benefit from extra skills that I dare say will outweigh def 99.999999999999999999999999999999999999999999999999999999% of the time (just for the sake of not blanket-stating 100 flat..though it probably really is 100 ^^).