Well, I'm not as experienced as most of the others on the forum, but it seems that LFs are useful in the very early game to expand to your first couple asteroids/deserts/terrans. Beyond that, I'm not so sure... I can imagine if you're super at Micro and you have the Sabotage Reactor researched you can use a squad of LFs to take out the antimatter of your enemy's capital ships.

It's not that they have no role, it's just that LRMS and Heavies can take them out so quickly, that you wonder why bother making them.

Sabatoge reactor is great for stopping Vasari and Advent fleets from using their supporting abilities. The problem is to do that, they have to survive my first point.

The only thing they're really useful for is taking out opposing supporting vessels, which have heavy armor, and flak, which also have heavy armor. They don't do too much to anything else.

Light frigs are strong against heavy armor, aka flak frigates and support cruisers. The problem is, LRF own them hard, and HC own them even harder and that's the majority of what people build. If any support cruisers get added to the fleet, they are always guarding LRF or HC and so the LF don't stand a chance.

So this leaves them with only a very small role, countering flaks. Considering most people don't mass flak there is usually no real reason to build many LF.

One counter to LRF spam is flak spam (at least to stall them, flak kills them slowly) so in theory combined LRF/LF spam may be better then straight LRF spam if your enemy is countering with flaks but I don't know how well that would work out in practice.

In the start of the game LF help you expand but once you get better ships like heavies and LRF the only use i would use them for is to maybe send them in first to take some of the fire while long range and heavies take them out, there kinda an extra sheild. Other wise they take your money and time and are a 100% wasteful.

Light frigates are good for decoys and targets for the AI's to kill while you focus on building actually GOOD ships