Here are some good overall roles for corvettes per each race:

TEC: Strong mid to late game attrition ship for the TEC, not uniquely strong at anything, worst ship in the game when it is compared to any other ship for damage/cost.  At the same time, its fast, allowing the TEC superior economy to directly impact fights from a distance that no other ship in the game can provide them with.  Good at anti bomber capability for late game. Tech level 4 on the military tree.

Advent corvette: strong opener enabling the advent fleet to hold their space long enough to get an economy up and running and/or put their opponent on the defensive.  The best dps/cost of the 3 corvette types.  Countered by large groups of anything, good at ensuring that large groups don't necessarily happen.  Tech level 1 to ensure that the advent have a strong early game defensive advantage.  The offensive advantage can be potent, yet against a competent defender it is not so much.

Vasari corvette: somewhere between the capabilities of the advent and the TEC.  Slightly later tech, tech level 3 on the military tree.  These ships are to help counter early advent and TEC rushes to hopefully prevent a forced starbase investment, and are good for mid/late game anti bomber capability. 

Honestly, this tech should be added to the game now that corvettes are all the rage:

All races get a buff to the armor and HP of their constuctor units in thier gravity wells, since these are usually priority target number one.  The advent instead would get decent shields for their constructor units.