This. Also, why does Sparrow or Twitter/Mac insist on inventing new widgets and putting buttons in the title bar and stuff. It really annoys me.
IMHO the "left category bar" of Sparrow and Twitter for Mac are pretty useful, given that (a) screens are much more wide than high, and stacking everything in horizontal bars is a waste of space (b) they move "categories" from horizontal tabs to vertical, thus making the Y direction for 3-or-more finger gestures meaningful and intuitive.
Nothing against the left category bar, but why not use a standard widget? Think Mail or iTunes. Also, selecting an item from a vertical list on the left and changing content horizontally on the right feels strange to me.