I'm not trying to be snarky, I'm really curious, as I tend to buy either A rated modules or D . Why would you buy a B or a C at all? Again, serious question as I'm assuming there's something I'm just not taking into account.
I know
@St0rmFury already gave an answer, but for activities where you're reasonably anticipating a rebuy (particularly experiments) a C rate can really minimise rebuy... particularly for fuel scoops as you mentioned re: cost.
Minimising expense on things like scoop, fsd, hull etc can be the difference between a 5m and 25m rebuy for larger ships... I've been known to go out in ships with an empty internal or two, just because I'm expecting to die a few times faffing about with Thargoids, so i strip my a-rate scoop and fsd to get those rebuy costs down... yeah money is easy but I'll still take 50m worth of rebuys over 250m for the same number of rebuys.
Re B rates... they're also very useful less for combat, and more for hazardous environments where there's gradual downgrade of integrity, whether that's clean- hauling corrosives or operating in high heat situations for extended periods... B rate can help.