There's no free lunch in orbit. If it dips into the atmosphere enough for control surfaces to bite then it has to boost back up again to avoid orbital decay. Either way it can't possibly carry enough fuel to do much of that.
> it has to boost back up again to avoid orbital decay
It doesn’t have to return to orbit. Switching from an orbital trajectory to a parabolic intercept path would be feasible, and open valuable tactical space.
As long as it can travel back to a US-friendly airfield. Which is probably a large number around the world although few which would support classified vehicles?
If a spacecraft is traveling at 4,000 m/s and it plows headlong into a planet, it has gained 4,000 m/s of delta v.
Delta v a measurement of a vector, not a scalar. The fact that its kinetic energy with respect to Earth is decreasing does not mean it isn't gaining delta v. In addition to the obvious (lowering your apogee) you can also use this delta v to change your inclination. It's not "free" of course- you need to do other engine burns to both put the craft into the atmosphere and then another burn to reestablish orbit once you're out the other side. But it isn't clear to me that this is less efficient than just doing the burns without the atmospheric assists.
> If a spacecraft is traveling at 4,000 m/s and it plows headlong into a planet, it has gained 4,000 m/s of delta v.
Yes and no. You've changed your velocity by 4km/s. You didn't gain usable ∆v, and you've just lost your spacecraft.
∆v means few related things. One, the amount of velocity change your mission plan requires. Second, the velocity change your craft is capable of using its own propulsion. Say you're plotting Mars orbit insertion. It requires (via [0]) 1.5 km/s ∆v. But you're a cheater, and decided to use aerobraking to bleed off (guessing a number) 1km/s of that velocity. This means you can complete this maneuver with a craft that only has 500m/s ∆v at the point of insertion. This does not mean the craft has gained ∆v.
You can certainly get delta-v with drag. You’ll keep bleeding energy from orbital (kinetic plus potential) to atmospheric heat, but that’s an entirely different thing.