I know what Spirit link is worse on paper, but that doesn't change the fact that it just works better most of the time in practice. It probably has more to do with the healing received than the damage though.
I fail to see how it's better in practice than it is on paper - except when there is a healing debuff that it can bypass (in which case it's game breaking). People think SLT is amazing, because it is very noticeable on your raid frames when it's used and everyone's HP is suddenly leveled off. However, when you actually think about the mechanics behind it, that health redistribution is really not giving you anything; it's still the same total damage to be healed. The exception would be damage mechanics where some people take more damage than other people. Most of the damage patterns you want to raid wall are massive damage to the entire raid.
90%+ of the time, Barrier is more valuable than SLT.
Almost every time someone dies from raid wide damage, it's just one person, or one person at a time. If a bunch of people die at the same time it usually happens only because of a failed mechanic, not damage/healing. What spirit link does is makes it nearly impossible for one person to die by themselves, which in practice makes it difficult for them to die period. Sure the numbers are bogus, but the effect works. It prevents people from dying which is much more important than simply reducing damage. Seeing Spirit link in effect is fairly obvious, not from the health bars, but from how many people (don't) die.
Also, spirit link is MUCH more effective when people are already dipping low than barrier. Cast barrier when people are in danger and they usually die anyway, cast spirit link and they survive the next hit. Barrier has to be cast before damage, including non-aoe damage preceding a raid aoe hit for it to work, where spirit link works both before and in the middle of damage.
Another thing you seem to forget about spirit link is that it makes just about everyone better. By evening out the highs and lows, you almost totally eliminate overhealing from most AoE heals, which GREATLY increases the healing your healers are capable of doing, usually far in excess of 15%. As a priest, simply eliminating that one guy (making him a valid healing target) who used a personal CD/healthstone to keep his health near full in a group increases my healing done by PoH by 25% on that group because im healing 5 targets with no overheal instead of 4, it also evens out crits to much the same effect.
90% of the time spirit link is way more valuable. It may sometimes make up for less overall equivalent healing but it saves lives. If all you are dealing with is flat raid damage in excess of what your raid is able to heal (and that is exactly what everyone is best at healing) barrier can be better, but even during that sort of flat raid aoe different classes and different players take damage and healing differently, so you end up with the largely uneven health pools that play to spirit links strengths.