Because, they're good against ONLY lrf's. They do anti light, and you need a lot of them, but not more ships slots than your opponent.
Consider this:
3 TEC Scouts vs. 1 Illum
Cost: 600 credits vs. 380 Cred, 60 Metal, and 55 Crystal Winner: Scouts
Used Ship Slots: 6 vs. 6 Winner: even
Damage (With percentages calculated): 18 dps vs. 13.7 dps Winner: Scouts
Hull: 1350 vs. 620 Winner: Scouts
Shields: 525 vs. 550 Winner: Illum(Ok, it's advent, they should win this category)
Production Time: 36 seconds vs. 26 seconds Winner: Illum
(Yup, production time is a drawback, but you can build scouts from the beginning, other players can't build LRF's from the beginning, and eventually, it should be scouts and lrf's vs. lrf's)
I'd like for the Counter to a unit to be more apparant, like, it should be the scouts do twice the damage to lrfs that lrfs do to scouts, but hey, that's the evidence right there.
The figures for Vasari and Advent scouts are similar.
I'm sure he was laughing. A bunch of scouts are the most laughable thing in the game, but they stop an opponent from making the best unit in the game while the guy with scouts is free to make as many lrm's as he wishes.
I said they were the counter, I didn't say they were a good unit.
Also, fighters are the counter to scouts because fighters are the only thing that can chase down scouts. It only takes 4 runs-ish with a fighter group to kill a scout, but it takes 7-ish to kill an lrf with fighters.
My point is that scouts kill LRF's much more effectively than fighters.