Seems a bit excessive imho - a frigate with better armour than most cruisers and the idea that a fairly average hive world in a backwater like calixis has dozens of them and that so do hundreds of other systems in the calixis sector doesn't gel for me personally. It also means you need a ton of ships to threaten a normal system which encourages players to counter such threats with huge fleets which are poorly handled by the rules.

Additionally with terrible speed and handling they seem completely unsuited to the sort of trouble I would envisage in a system primarily coming from ultra mobile pirates in raiders, smugglers etc.

These ships are basically semi mobile defence points for fortress worlds?

First; Calixis is not really a backwater sector.

Second : having a strong detterent against interstellar invaders is the point of a system defense, especially when knowing the doctrine of the Imperium (ie, the world defend itsef and must hold on the time reinforcement is scrambled from the nearest worlds).

Third : a monitor is slow, not nimble, use short range weaponry, and can't leave the system. The least it can do to be useful.is to be well armoured and well-gunned.

Fourth : A true vessel of war can dstroy that whitout too much an hindrance. Lances, torpedos, bombers squadrons, and even RAW macrovolleys of doom will much them quick and easy.

Fifth : it's the sort of thing you find over civilized, well established imperial worlds, along with space stations and minefields. Not really in the Expanse where you are supposed to go to discover the undiscovered.

Sixth : They are the the local enforcers in said place in space. Smugglers, pirates and other neerdowells, even those system bound, won't use indeed that kind of ship, not suited to their needs.

The existence of so many system defense monitors has some interesting implications that if you're not building a warp-capable vessel build times are a lot faster. That probably comes from not having to make sure you're building a ship that has Emperor-pleasing geometric designs than Chaos-worshipping non-euclidian designs. Your choices are really limited when you have to make sure there are absolutely no 6, 7, 8, or 9 sided things anywhere on an entire vessel.

I honestly don't see pirates as the penultimate threat to a system. Having fast raiders swoop in, and take their share of the booty is irritating, to be certain, and bloodying the Imperium's nose is a good way to provoke them into breaking your jaw (possibly with a bolter ), but the planet is still there, and can "make more". Depending on what you're taking, what quantity of worthy loot can they take, when compared to the Universe Mass Conveyors that might be used to haul "small" supplies of foodstuffs, or what have you? Pirates are like mosquitoes; an irritating annoyance, nine days out of ten, and that tenth day they give you malaria (thanks Nurgle ). What I see as the real threat would be an invasion force, one that doesn't plan to leave, or leave the people in service to the Emperor. Whether it be Orks, Nids, Choas, or what, a fleet of battle-capable ships, coming to take your stuff, your people, and your loyalty, to deprive the Imperium of all that a world can offer it, that is what they don't like, and that is what such beastly monitors would be for. As was said, it can take a while for reinforcements, if they EVER come, and such worlds need to be able to protect themselves, till then. With Orks who won't quit, Nids who will replenish from your losses, and Chaos calling in backup from everywhere/when, you'd need powerful defense forces, and we can't all get Ramillies Star Forts, or Blackstone Fortresses. When you need to be able to make sure any world can be kept in the fold, you build something like this, to make sure that's where it stays. Pirates, you know they will just swoop in, grab what they can, and run, and you sort of let them, and make it expensive, if you can, but invaders are the problem, and you need to slow them down, and stall them out.

I didn't mean pirates were the ultimate threat to a system but rather that small nimble craft would be the majority of trouble on a day to day basis - would you really maintain a fleet of sector ships at huge expense of resources and maintenance when the chance you would need them was maybe 1 in a thousand years?

Additionally beyond BFG these sector defence ships don't really feature in the fluff and definitely not to the levels that are implied in Hostile Acquisitions.