A good argument for it would be that he was the pioneer of free-verse and opened the door for most everything we now consider modern poetry. He put his soul in words at a time when precision, concision, and modesty were the rule. This brought poetry to the masses, rather than leaving it as a resented craft of snotty elitists.
This is not to say that he's my favorite... but he did do some amazing things for making poetry more accessible to the masses rather than leaving it as an artisan field, and I think that's the sort of thing that was necessary for poetry to thrive as a true literary medium rather than just a teenage girl's ranting medium through the technological boom of communication--with the introduction of cheap paperbacks, radio, television, etc, no one was reading gold-leaf books anymore or memorizing Longfellow outside of the 19th century.
Longfellow seems to have died out a bit and Whitman is heralded. He was ahead of his time a bit, but necessarily so. ( :
