...and I haven't yet said a single dretch can tear down a stack of human turrets. Not once. I outlined the method you could use to bypass walls of human turrets with a 'sacrafice play' easilly, because until a DefComp is set up all turrets in LoS to a target will focus on the first target any single turret spots. Sure, you're multiplying the damage output, but until you get the chance to set up multiple turrets your base is very vulnerable to one dretch letting the entire alien team in.
Seperately, just to prove
a dretch can take down the default coverage for a base entrance on an existing map just unzip that .pk3 into your Tremulous directory and type '/demo tenth' to see. The tenth references various demos I've been recording to demonstrate various ideas and concepts such as visual explanation for why you don't place turrets off ledges, etc. Just under 20 seconds from spawn to side-alley turret destroyed as a Dretch. Other dretches going that way wouldn't have even been shot as they passed since it was targetting me.
And for the two tyrants I was dancing with until I was down to 30 lasgun rounds, one of them was the high scorer on the server, the other was 3rd place. Neither was bad as chopping down humans, I just had a helmet, and kept ducking and pausing (yes, pausing, it's amusing to see a Tyrant zoom past where you would have been if you hadn't stopped) so I wasn't just an easy-to-track circling human target.
The reason I'm not moving the discussion to the territory both of you want, a well-built human base, is if the point was to fight aliens versus a well-built human base the maps should start with well-built human bases. The initial fight, where the telling handful of alien frags can be obtained that grant access to higher alien classes very early, happens within the first minute of a map. Not in the last minute. I'm debating the balance based on early-map situations before humans can 'tnak up' and a basic flaw in the damage-math (headshots being too powerful) allowing for metagaming (the idea of playing purely to win even if the playstyle looks strange, isn't what was intended, or uses knowledge you shouldn't yet have. For example knowing that trolls are vulnerable to acid and fire only, when your PC has never seen one before in a D20 game) to provide a dramatic increase in ability over the designed gameplay.