What they're really good at is mopping up swarms of small ships. They can pretty easily eat a whole wave by themselves, but you do have to watch out for any bigger ships, where they are obviously powerless. They're nice for softening up a system before you send in your main attack force. Sometimes I'll just send in a few hundred to a hostile world and put them on FRD, and then come back a few minutes later to bring my actual assault force through to mop up the guard posts. What I tend to abuse them for the most is beefing up my apparent firepower on a world. If I need to move most of my strength out of a system, leaving a few hundred shredders hanging out will keep the AI wormhole campers from suddenly deciding it's a good time to pop in. That's risky, of course, because if even a single guardian comes through it can easily have its way with the world unless I've built up a good turret defense.
So no, they're not terribly out of whack, although sometimes they do confuse me about my own ship numbers. I think I have a great force to take a world because it shows 500 ships there on the map, but then when I look at the world, I find that it's actually 499 shredders and a single fighter.