If you browse the forums here a bit, you'll find numerous discussions on this topic. I won't reproduce those here, just a quick two cents:
- Interpretation 1: The various roles and subroles are heuristic devices for your convenience, nothing more. You make small ones, and you know those are throwaway; you see enemy small ones, you know he's scouting / escorting, not seriously threatening. I like to build a bunch of ships < destroyer sometimes, just feels better.
- Interpretation 2: While a "bigger" ship might be "better" in a fight, they might not be "better" all the time; the smaller ones might be "better" for escorting, scouting, harrassing; at least economically speaking, if resources were ever an issue (sadly, they are not), but building time may sometimes be.