Regarding RSD's further questions. First off, back to the high tech case:
While it may seem silly to keep a bunch of fusion gun armed troopers
around for crowd control, remember that all weaponry is relative (this is
a problem that I have with the standard Law Level system, as you will see).
There are both criminals and otherwise law-abiding citizens around in the
USA who have fully automatic weapons or weapons that can be easily converted
to such, as well as all kinds of explosives. As a result police where vests
and have access to shotguns and SWAT teams to handle such threats.
Translating this to a high-tech case, a relatively libertarian future
society will have some citizens who are well armed for criminal purposes,
for self-defense, or for recreation, and the police and internal security
will scale up to deal with this.
Possible Imperium organization: the IPA - Imperial Plasma-Gun
Association. Plasma Projectors don't kill beings, beings kill beings.
On the other hand, a government sufficiently restrictive and intrusive
to prevent the citizenry from being well armed would probably have an
internal army like the Soviet MVD for all sorts of uses. Not necesarrily,
though, as the case of England points off (not perfect, but they do better
than we).
On the low-tech side, a small high-tech palace guard is very likely,
especially if the rulers don't trust the local people, but I think that
the bulk of the army would still be low tech, simply for price's sake
(in my universe the exchange rate is more slanted against low-tech planets
than the Trillions Credit Squadron table). To be honest, I also do this
because I like Byzantine political struggles (between the low-tech militia,
the mercanaries, the mid-tech imperial wannabes, the government trying to
keep them from shooting each other, security forces looking for treason,
etc. Yes, I do occasionally play Paranoia).
As for the armed forces being turned completely into triphibious grav
vehicles, I stick with Robert Heinlein (Starship Troopers) and say that
the infantry will always be there. They're the cheapest self-aiming
self-controlling weapons available.