If you're still talking SR, once slotted up in the three passives and the three toggles, you'll be capped on Defense Debuff Resistance and you'll never have to be worried about defense debuffs again.
If you're talking FA, a little bit extra will help a little bit but you will see cascading defense failures, and see them hard, from some factions.
To more bits on SR (not sure if already covered.)
The normal softcap is 45%. For incarnate content, since the enemies there have a 13% tohit buff, 58% is the softcap. Anything over 58% on an SR user is a waste. Go for more HP and more DamRes. The two coupled make your scaling resists rock.