See the 'should there be subjobs' thread for more people railing against /NIN supremacy.
While I feel that it's perfectly reasonable to have a tanking class that mitigates damage (PLD, to a lesser extent WAR) and one that avoids damage (NIN, or evasion/counter abilities like SAM or MNK have), I don't think shadows are necessary. I would not begrudge them being given to certain job(s), but the current state of affairs with the subjob system (or a skill-tree system as many have mentioned) combined with the great perks shadows have over DEF is really contrary to the spirit of variation within the game.
It's too late to change it now, but I wonder if /NIN would be nearly as omnipresent if Utsusemi and Dual Wield did not come from the same sub, making it often both the strongest defensive and strongest offensive sub available. Would there be more variation among players - the extra safety of shadows vs the extra dagger for SATAWS as thief, for example? Would things remain largely the same, with DDs enhancing their damage to the exclusion of all else and just making the lack of shadows the healer's problem?