ive owned my flareside for 6 years and have yet to build a single house. i have hauled one or two 4x6 sheets of plywood from time to time...they fit fine if leaned against one bedrail. ive hauled a 3,000lb roll of galvanized sheet metal, a piano, litterally hundres of pallets (i was the only one in high school with a pickup for about a year and a half), landscaping rocks, garbage, engines, axles, people, 1500lbs of batteries, and my girlfriend and i camped in it (under the topper) for 5 days on our way through canada.
flaresides are far from pointless. they have LESS space...but not by much.