I just don't get them.

They're T1, very effective against pretty much everything. They seem to break the pattern of LRM kills LF - fighters kill LRM - flak kills fighters early tech race. Now I just spam Corvettes and they get the job done very effectively. 

What are they?

They move like strikecraft. They have more weapon types than Frigates and Cruisers packed into a tiny little ship. And they have some passive disruptive abillity on top of that.

Why don't Frigates and Cruisers get multiple weapon types rather than the smallest ship? The current ship lineup doesn't make sense.

I think Corvettes currently have too much packed into one little ship and that they lack a clear role. A logical early game role  for them would be anti-LRM.. or anti-"any ship that only fires forward". Good against Frigates but paper against Capital ships with multiple weapon banks that you can't fly around.

I think a single forward firing weapon would be quite enough for them and that they should be mostly anti-frigate. Give the extra weapon banks to Cruisers instead because it makes more sense and lets them swat Corvettes as a T5 answer to them.

I like the idea of additional early game caps being the answer to something.. i.e. corvette spam.