M Def is a big factor for protecting yourself in pvm.
1 mdef is equal to 1 less damage, but before every multiplier is applied.
Most mobs have damage multipliers ranging from 2x to 8x. The general rule of thumb is that mobs that deal higher damage have higher multipliers. This means mdef is very valuable to reduce damage you take at rank 8 content and above.
Having something like 2k mdef or pdef means most magical / physical mobs will hit you for 1 damage in pvm. In pvp it’s still nice to have but not that important because many player skills have very high base damage without any multiplers (esp rank 8 skills). That’s why most pvp-focused builds will go for very high CON instead of high pdef / mdef to guarantee survivability.
SPR builds can be quite good but they require careful planning because there are only a few skills that scale off SPR which are actually worth building around them.
Sadly, building SPR for Hexing is not worth it because reducing enemies mdef is much less valuable than building your own mdef for pvm. Like I’ve explained before, player skills have very high base atk values and IMC compensate for that by giving mobs unrealistic high pdef / mdef values that are rarely achieved by players. Mobs past rank 8 content can reach 2k mdef easily and thus reducing ~500 of their mdef (in a full SPR Hex build) would mean very little.
So in short, increasing mdef = very good against mobs, but bad against other players. Decreasing mdef = bad against both mobs and players unless its a mob reducing a player’s mdef (and then attacking them).