It's true that Benefit street doesnt really have a full service neighborhood associated with it, but the great thing about Providence is that almost everything is accessible by foot. On one end of Benefit, you have University Heights which has some of the retail staples (no pun intended), and on the other end you have Wickenden St. Thayer is close to the east, and downtown close to the west.
And as far as the "only really fully realized neighborhoods" on the East side, I would have to include Wickenden/Fox Point, which is where I live. I have a ton of great restaurants within a 3 minute walk, and it definitely feels like a neighborhood. Plus I can walk downtown easily (walked to RiRa last weekend in about 10-15 min).