I've been doing a bit of thinking about this FOBS+HGV combo and I've come to the conclusion that it's fook'n brilliant (yes, that's the term). Unfortunately, the popular media won't give you any sense of this because the public figures capable of doing an analysis like the one I'm about to present are in the "arms control" community, and have every incentive to downplay the technical brilliance of the weapon. Their opponents, on the other hand, know only to shout the buzzword "hypersonic!" to frighten the masses. They don't understand any of this.
I'll first start by confusing you further about FOBS. In popular depictions, FOBS is contrasted against the arcing trajectory of a ballistic missile by depicting it as a circular orbit. If you'll remember back to your high school physics, orbits need not be circular - orbits are elliptical. Circular orbits are just a special case of elliptical orbits where the semi-major and semi-minor axes are equal. For very good reasons, no designer would ever consider a circular orbit for a weapon like this - because unless the HGV bus expends a lot of fuel to change the trajectory of the orbit (called a deorbiting burn), the weapon will just keep going around the circle for the next umpteen decades.
Elliptical orbits allow a very clever way around this by "deorbiting" at zero cost. How? By having the periapse (the point on the orbit closest to the Earth) be within the atmosphere. Nobody would launch a satellite like this because it would just burn up and fall, but it's exactly what the doctor ordered for an HGV. You program the orbit such that the insertion point of the HGV (not the point above the target, but a point along the circumference of a circle centered at the target with a radius equal to the glider's range) is at the periapse, within the atmosphere, where it will be traveling fastest. You launch the rocket farther away from the periapse, where it would complete a fraction an orbit around the ellipse. When it reaches the periapse, the HGV will "catch the wind" so to speak and glide toward its target in excess of Mach 20.
If you're even more devious, you select a trajectory where the apoapse (the highest point of the orbit where the payload is traveling slowest, i.e. where the HGV is most vulnerable) is above the Antarctic. I can comfortably predict that the penguins won't be firing interceptors at the warhead. You can also change the inclination of the orbit at the apoapse at the cost of some fuel, which would change the approach trajectory of the HGV while it's still in orbit. This is kind of a big deal, since the only way to change the approach trajectory with an ICBM is to launch from a different position, i.e., by launching from a truck or submarine. But this allows a heavy, liquid-fuelled, silo-based missile to approach from multiple angles while still launching from its fixed position. Doesn't this sound like a great upgrade program for the DF-5 to you?
So, to summarize, you've sent an HGV along an unpredictable trajectory and inserted it around 2000km from your target, traveling at around 8km/s and with zero cumulative thermal load. That means you can engineer your glider to withstand these horrible thermal conditions only for a short distance, and there's less scope for drag to slow it down than if it had to glide for 12,000km+.
I hope this has given you a feel for why this is such a brilliant idea. That's GG, right there.