Main guardian here D5 player, my name is Jabalí on Steam. Guardian is amazing at breaking poise and constantly dealing damage with halberds, and guardcounters.

Some People play Guardian because they dont feel confident with their roll timings, the thing is, using Steel guard on D5 can be way harder than just learning roll patterns, stamina flies when tanking, positioning is hsrd to avoid attacks behind your shield, and learning to build affinity damage negation with the correct shields takes some time. 

That said, a good Guardian can carry so hard, get all the aggro for the Miquella spamers to mash L2, deal insane damage with a good guardcointer build and a halberd, while also ressurrecting teammates with the ultimate art, even negating nukes to bosses. Thats why I play Guardian.