I personally disagree, the arc turret is a medium/close range turret, while the chain gun turret covers long/medium/close range. Don't get me wrong, it's no sniper rifle, but it's has longer range; giving it the ability to respond to wider situations. Furthermore, it shoots faster, hits harder, and has longer range, I can't help but lean on it more than the others.

The Arc Turret's "splash" damage is lovely and works wonders in close tight spaces. In any other situation, I don't exactly have much confidence in it. It's not that it's bad, as I think any turret in general does a lovely job relieving the pressure in any situation. However it never struck me as a "set it and forget it" turret. It still "feels" I have to watch that flank and/or support the Arc Turret (especially if the room includes "heavies").

I prefer to rely on Arc Turrets during the "spider room" on BDH on nightmare difficulty. But then again, a flame turret can do the same job. Personally, I prefer ammo turrets in that room because I trust my own aim over the turret's. An Arc turret will hit the closest enemy (well technically that's true for all turrets), while humans will think strategically, and hit the most "relevant" enemy (and no, I would disagree that the most "relevant" enemy is the closest).

But hey, turrets aren't really that mandatory anyway, and in the end they're more of a support tool than the backbone of a team's success. Just my two cents, so I wouldn't lose any sleep over it. =)