Those turrets probably aren't gonna set the world on fire by themselves, but I'm willing to bet that the weapon that gives you the ability to put down the turret in the first place will have skills that complement it. I've seen other engineer videos that feature a bunch of single target snares, AoE snares, and AoE slows. A small but steady stream of damage while keeping the target pinned down and harassed will undoubtedly be what the turret is for, not as the cornerstone of your DPS.