The other engineering grades would matter if engineering were balanced as it should be: side grades and specialization, rather than gigantic direct upgrades. Factory-spec would be the reliable generalist choice, G5 would be the very specialized choice, and G1-G4 would fall on the spectrum between them. E and C rated modules would largely still be niche at best, usually just something someone temporarily has while they're progressing to better stuff. D is the lightweight choice, B is the heavily armored choice, and A is the high performance choice.
With engineering as it is, though, even A-rated factory-spec stuff is generally trash compared to fully engineered stuff. Buying a new ship is a someone mixed experience, as a result. You're excited to have a new ship, sure, but it is going to be garbage (relatively speaking) until fully engineered, so you've also purchased a huge to-do list. Experimenting with different builds and setups is also a bit of chore since factory-spec stuff won't even give you a good feel for what it -could- be, and grinding a bunch of engineering upgrades for a build you just want to experiment with (and may not keep) is downright tedious.
Edit: if engineering hasn't largely made power management a joke, C-class COULD be the "best-bang-for-your-MW" option. With the game as it is, though, I'm not sure what niche C-class could realistically be given. I don't really have any ideas for E-class, but I don't mind a single class playing the role of "cheapo crap you really should replace ASAP." It would be nice if the performance difference between the classes were a little narrower such that their respective specialisations could be a bit more attractive, but that issue is so miniscule compared to the engineering catastrophe that it's hardly worth putting too much thought into at the moment.