That all is true, I often try to use this turret whenever possible, but it's rarely worth it. There is also one more downside of it you haven't mentioned: it's dangerous to nearby allies. By making this turret in front you endanger all barricades nearby and people will get toasted whenever an enemy gets in close range. The only case when this turret is actually effective in siege is when combined with AOE attacks, like GL, enemies are slowed down by fire and this helps too. It's all good but you never get to have enough AOE weapons to make it up for players with regular weapons who can't support this front.
The ammo is honestly not particilarly small, the thing is the turret wastes way more ammo than it needs to, especially when there are small enemies coming 1 by 1.
In my vision I imagine this turret having many uses in current meta, if fixed. Continuing my "advanced" previous ideas, here is one I think would help: addition of target preset - settings in a form of abilites available for engineer.
The turret would have 3 modes:
1)-current one, to waste ammo as fast as possible;
2)-new mode: only target rare enemies (anti-reanimator);
3)-save ammo mode: limit of targeting based on number of burning stacks on enemy (1 for smallest enemies, 2 for reapers, 3 for rare enemies). This means turret only won't target the enemy, not counting AOE.