-Resistance diminishing returns begin after 20 Res. You get 3~4 fire defense points per Res stat below 20 Res. After 20 Res, 2~3 fire defense points per stat. Physical defense remains constant at 2 defense points per Res stat.
-Where fromsoftware screw things up: NO matter the number of Resistance, SEVERE diminishing returns begin at 120 overall defense. After this point, Resistance only grants 1 fire defense and physical defense, the same as increasing any other stat.
-Humanity also grants a free defense boost, capping at 170 overall defense. (on an SL 120, defense increase stops at ~70 humanity)
-On an SL +700, overall defense gets as high as 200.
-If the defense cap was higher, say 200 overall defense, and if other stats didn't increase overall defense, then Resistance would be a useful stat, especially for pure stat builds and tanks.