Pros:
Higher defensive armor tier (Mystic, Initiate)
Less food usage when training
Mithril Defenders
Adamant Gloves OR
Access to training at Yaks
Cons:
Mithril/Initiate is still fairly weak; and you'll probably get hit anyway in PvP
4 and 3/4 higher combat levels than a 1 defense pure, or roughly 12 strength and 5 hp weaker
Mithril Defenders are annoying to get because Cyclopes hit pretty hard on 20 defense players
both have advantages.
I like Initiate pures, as I had one back in 2007. Staking wise, I seemed to destroy mains and pures around my level (I was 13 prayer, so still low combat).