Light Frigs are supposed to be awful for actual engagements. You get them from the start. If LF rushing was a legit tactic, this game would be COMPLETELY 1 dimensional. As it stands, two units are available form the start. Scouts. LF's. Yes. LF's suck. But they DO help with early expansion.
The place where the problem enters is that everyone at some point gets support cruisers yes? I mean, eventually you're going to, right? Well, what counters them? Anyone? Light frigs....yay. So you surround them with lrf's to stop from losing them. LF's do kill off support cruisers effectively. Thanks to lrf's though, they never get near them.
On top of this, lrf's get the 3rd best damage multipliers in the game after only HC's (Best in the game, but late game tech), and capital ships (At best support to your fleet, not ur actual fleet). Link those two issues together, and you have a game where people will rush to lrf each and every time.
How do you get around this? Well, there's the little trick of the scout rush I have shown people (raise ur hand and say you've used it and I'll give you karma). Only problem is if you get hit with a Seige Drop (marza or desolator rush) or get a starbase sent your way. Get's hairy there. Need scouts watching to see if ur enemy is gonna do that. Then you know to switch to lrf's.
Which by the way. Why would people do starbase rushes and Marza rushes if the lrf spam is the most common strategy in the game? I mean, in all seriousness, the best counter to a starbase rush in the very very very early stages of the game is a lrf rush. The best counter to a Marza rush is a lrf rush, because you can actually kill it.
Last bit's a little off topic, but still, points made.