I've found that the TEC has two different Attrition style plays that work. Against Human opponents, the key to victory is an aggressive offense that keeps the enemy rocking; never let them get set! However, in allof the RTS games I play, whether it is the Stronghold series, Star Wars: Empire at War, American Conquest, Gal Civ, etc.; I have found that the AI is superior in the short run to a Human opponent. Speaking simply, the AI has the ability to multitask, and it always knows the current strength and disposition of my forces. I've found that the most effective way to defeat an AI, is to reach out and gobble up as much territory as I can at the onset of the game, and then dig in and let the AI break over me like a wave. I keep strong static defenses on my fringe worlds, with backup fleets sitting in the phase lanes a gravity well behind. This always fools the computer into attacking with an understrength fleet, and allows me to totally annihilate it with a relatively small one. Meanwhile, I build up and invasion fleet and bide my time. About 90% of the time this tactic leads to an economic stall on behalf of the computer, as it must continually rebuild its fleets, and cannot reinvest in its economy. The end result is a floundering AI that is ripe for conquest. If I lose a battle, then I do so Bunker Hill-style; the AI, though victorious, is so badly crippled by its victory that it cannot continue its onslaught.
Humans, on the other hand, can employ tactics; therefore, they are much superior in the long run, and much more dangerous. Y'all are correct that the best defense is a good offense; it's the only way to win an RTS game against Human opponents. 
As far as static defenses lacking worth...they can be enough to tip the scales of a battle. Hangars are especially effective with their strike craft and flak busting capabilities. Overlapping fields of fire can effectively dismantle an enemy strike craft attack, while the hangar-based strike craft can accent a defense fleet quite admirably. Despite the classic feel of gauss rifle turrets, I have never seen them effectively deployed, and rarely use them myself. I rarely use repair platforms due to their tactical slot requirements. Instead, I employ scores of Hoshikos as fleet support.
As for the Envoys...they make pretty good scouts. They're tougher due to their immunity capability, and are not as conspicuous as a scout or a warship. "The only good form of diplomacy is a fully-charged phaser bank!" Scotty Star Trek
Overall, I agree with y'all; the TEC is a vulnerable faction early in the game. However, it is probably the strongest late-game player due to its ability to out-produce the other factions... 