It is useful if you take the perks that are specifically for making dual wielding useful. Stacking elemental fury or slow time on top of those abilities certainly makes it viable. However, anybody playing on legendary or RPing with realism is probably going to avoid dual wielding attacks; preferring instead a shield, spell, or staff to provide defense. Hell, even dropping the left weapon so you can block is notably smarter.
It's risky and difficult to make practicable. Once the player has both tiers unlocked for Dual Flurry and has deep stamina pool, then enemies can pretty much be stun locked. Fighting groups or just generally tough enemies is made easier once the player has Dual Savagery and maybe even some fortify one handed enchantments. Having a decent armor rating is a plus.
I think dual wielding is overpowered, even on legendary with the right perks/shouts you can non stop power attack flurry, staggering your enemy every time, and perks that make you put off crazy damage, i had 2 Skyforged steel sword putting off around 70dam each, but when power attacking with the right perks they were putting over 200dam per hit out, staggering every time, if they didnt die the first flurry i would just give them a second, i ended up starting over because it got boring killing everythig so easy.