Yeah, some weapons and shields can be equipped by classes that don't have any skill in it. Combat skill only matters when you actually try to use a weapon to attack a target/block a target's attack on you, but the stats on those items are always active, regardless of if you try to melee with them or not.

Genbu's shield for example, is equippable by WHM, BLM, RDM, BRD, SMN, BLU, but only WHM and RDM gets native shield skill. However, the 10% physical damage reduction, the +10 evasion, and the elemental weakness/strength on it will always be active for all of the jobs, even if they're not even engaged to the mob.

I also wish I could use this shield on my other non-mage jobs :(. 10 evasion would be really great for a thief!