I don't think that's true. In the wormhole segment of TMP, Ilia tries to use navigational deflectors to keep the asteroid from hitting the ship, but they're inoperative. That would seem to be their normal function, established canonically.
I'm referring specifically to the navigational deflector beam
, which might be related technologies but might not be directly involved in the act of deflecting. The plural term "navigational deflectors" is used several times in Trek, and e.g. seems to provide wide-angle protection against the weapons of the Montagues and the Capulets in "The Outrageous Okona". That's fine and well - "navigational deflectors" would simply be the same as all other deflectors, protecting the ship from all directions, only they'd be the weakest variant.
The TNG Tech Manual also makes a distinction between the deflectors and the deflection beam. The former plays a canonical role in all-around protection; the role of the latter is not clarified on screen.
I'm going to call bullshit on the Enterprise being unable to blast out of orbit on Delta Vega if their attempt to regenerate the warp drive fails.
Good point, and an excellent chance to bullshit our way out of the jam, too.
Ahem, let's see... Kirk says that bit in two pieces: "We'll be trapped in orbit here" and "We haven't enough power to blast back out". The two need not be literally connected: if the ship won't have enough power to reach civilization, then she's going to be trapped somewhere
, and "in orbit here" is as good an option as any.
That is, the act of entering orbit may not be the one that traps the ship, and the trapping itself does not consist of the inability to break out of orbit. The act of fiddling with the power packs may be the dubious step, and the results will move the next-nearest base to the "centuries away" category, meaning the crew could just as well spend the rest of their lives on the orbit of the cracking plant planet.
The same thing happens in "Mudd's Women" the the ship is dangerously low on power because of the burnt out dilithium crystals. What, no spares?
I don't see the objection. The plot already featured our heroes exhausting their cache of spares, after all.
Sure, Scotty seems to think it unusual that only a single crystal is on line towards the end. But in the teaser, the crystals are being used one by one, as if one were the spare for the previous one. Taken in the wider context, it would seem that somewhere between one and four crystals are needed for moving the ship, and all the rest are spares. That's details, though: the episode itself goes to the necessary effort to establish that spares are gone.