The real bottom line is, you get twice as much firepower out of LRF's before they get blown up. But they're damn easy to blow up.
If you're careful with them, you won't lose them that easily. As I've said before, the key to good play is to be proactive, and stay one move ahead of your enemy. You don't lose your harassing LRF because they're already jumping out when the defenders arrive, and with carriers this is even easier to achieve.
I've had some great harass wars with enemies before, and I can tell you that it's somewhat rare to actually get a takedown on a carrier because a good player always keeps his escape routes open and won't allow himself to get cornered. Carriers are really tough, too, so you need to pound them with a lot of damage to take them down effectively, and with good kiting and fleet movements that's much easier said than done.
The problem with LF, as I already stated, is that they have to stick around and stay firing for much longer to get their kill. This gives defenders more time to catch up to them and drive them off before they deal serious damage. Their speed makes them harder to catch, but their low damage also means they're less dangerous to begin with.
The idea of using LRF is to lean on that high damage. Get in, blast your target, and get out before the enemy can do anything about it. How is a LRF going to die if it's already retreated before the defenders get there?
Assuming the LRF's made it to their target undetected. PSIDAR, anyone?
You'd need 2-jump PSIDAR to have any real benefit; good scouting gets you
way better intel than 1-jump PSIDAR. Personally I think 1-jump PSIDAR isn't even worth researching, since you really only get a 10 second heads up which in this game isn't worth a lot, and 2-jump PSIDAR is basically just an alternative to active scouting for inconveniently large empires. In any case, someone who has undefended flanks either needs to be actively scouting, or be prepared to write off some losses if he gets blindsided.
If he's got intel, your oppponent is preparing his defense before your harassers even arrive. This means bringing nearby units to that front-line to defend, and producing new units near the front-line (good players
will have frigate factories near every front, with repair platforms nearby, so forget about cracking them with a small harass force). If he's assembled before the harassers arrive, then it doesn't matter what units you have, they're doing an about-face. If he isn't assembled, it's about getting the job done before he arrives (and having your own counter-intel to be able to coordinate your retreat before he shows up). This is where LRF excel, they get the job done that much faster and give the enemy less time to react to you.
Okay. Fine. You retreat. And guess what, the defenders simply pursue you. You can't retreat from something that's faster than you.
You got a grand total of two units that are faster than LRF by enough margin to actually close distance: LF and scouts. I've already stated that LF are the correct unit against carriers in this situation, but the problem with light frigates against long range frigates is that you need to outnumber them 2:1 to have a good shot at beating them, which is completely impractical. This leaves you no recourse but scouts. Back in 1.181, scouts pursuing LRF was just as effective as LF pursuing carriers, and it was not uncommon to see roaming packs of scouts used both offensively and defensively to pressure LRF. If they were still what they used to be, I'd agree with you and write off LRF as unsupported harassers because they're too vulnerable. This is why the LRF issue has become a hot button again in 1.19, because the scout nerf neutered our best counter!
Now, pursuit over multiple gravity wells is a much more difficult proposition than pursuit over a single gravity well, and it essentially comes down to the jump. If you want to do a disorganized jump (the retreating enemy will do this) you can actually clear out very quickly and the jump itself is a negligable portion of travel time. If you
do not want to break formation and want a group jump, this means you have to both wait for your slowest units to cross the gravity well (you lose all speed advantage) and you need to line up, which takes even more time. This means if you want to successfully pursue across gravity wells, you need to do a disorganized jump.
Disorganized jumps work very well if the enemy doesn't have a substantial lead on you, but if they had some intel and started their retreat before you arrived, chances are you have a full gravity well of distance to cover to catch up. There's a fairly good chance they will elude you, reach friendly territory, or rendezvous with reinforcements before you catch up. This makes your job as defender very frustrating, and this is why good play is about being proactive. You can't catch up to someone who knows where you are and how much time he needs to successfully retreat, you need to be ready for him and deprive him the time he needs to get his job done. This is why LRF excel; they get the job done faster, so you need a smaller window of opportunity to successfully harass.
In any case, if fleets are big enough and we're talking about escourted carriers (which is to say, the classic LRF/Flak/Carrier fleet) then this is effectively a moot point since the only fleet you should send against such an opponent is LRF/flak/carrier or perhaps HC/flak/carrier.
assuming ideal circumstances for the LRF and non-ideal circumstances for the opponent. The reality is that the opponent will likely see you coming and MIGHT already have countermeasures in place.
Look before you leap. If you jump into an ambush, it's your own fault and it's not going to matter what units you have.
The reality is that your LRF's may not have anywhere to run to when they have to run--if they have to phase-jump five times to find a starbase to hide behind, they are going to die.
Again, your own fault if you let your LRF get stranded like that. Good players are proactive, and they don't let themselves get trapped like that.
If you're thinking ahead, 5 jumps isn't actually an impossible escape distance. Presume you have a 1 gravity well advantage, you'll get at least two jumps before the enemy closes, which means you have enough time to dispatch reinforcements from the other direction to rendezvous and make the enemy back off until his slower units catch up, which should let you escape.
Of course, going that deep into enemy territory is quite risky since all he needs to do is put up one well-placed PJI and you can be screwed regardless of your unit type (although bombers can blast the PJI while the carriers keep fleeing, so you have a good response there)