Stoutland's usefulness comes mainly from synergizing Stoutland with its Allies and Partners:
-Sand Rush Stoutland appreciates having Sand Stream Hippowdon around and can be a real pain for Excadrill if it's Jolly Stout' against Adamant Exca'
-Scrappy Stoutland is a definitive Revenge-KO Pokemon with Retaliate and can wreck Sableye and Spiritomb's natural lack of Weaknesses with Superpower (more often used on the bulkier Spiritomb)
-Intimidate Stoutland makes many Battles more survivable (even if slightly) for Pokemon with lower Defense compared to their Sp.Def like Cryogonal, Mantine, Mr. Mime, Kecleon, Hypno, Gallade, the Hitmon-trio, Flareon, Slowking, Grumpig, Roserade, Mismagius, Jellicent, Ninetales, Tentacruel, Muk, Venusaur, Politoed, Ludicolo, Stunfisk, Dewgong, Jynx...need I continue? There are so many Pokemon that appreciate coming in on an Attack debuffed Physical Attacker (especially because it can force a switch).
Stoutland's lackluster Stats aren't that lackluster. Stoutland's Attack ties Dusknoir, Flygon, Zebstrika, Throh, Sawsbuck, Klinklang, Tauros, Rapidash, and Sandslash, so it isn't too worthy of asking for more with that kind of company. Speed isn't the best, but Sand Rush cranks it into overdrive. Intimidate, as jesusfreak94 pointed out, raises your bulk in a small, yet significant way, allowing your Defense to be pseudo-buffed.