The fairy is effected by SS now, so that should be less than 3 seconds. How much less, I don't know, but around ~2.5-2.7s isnt unreasonable. And you can also use Rouse every 60s for 20s (40s downtime) to buff the fairy 40%, which if using Eos is a mssive healing buff when partnered with Whispering Dawn, and Fey illumination. Whisering Dawnbeing 50% more potent than Medica 2. Most people use Selene however, which brings Fey Wind to the group, a pretty large dps increase every 60s for 30s (30s downtime if used efficiently). More to the point, a Sch doesn't need to stay in Cleric Stance to deal a good amount of damage. 4 DoTs (5 if you use a very weak miasma II) plus Shadow Flare adds a large amount of dps, and easily allow you to put these up and then swap back Shield/Heal, then go back to Broil spamming until refresh. Whm does have 3 DoTs that tick for nearly the same potency as Sch DoTs, but this doesn't include Shadow Flare, and they also tick for much less time, on average, requiring much more frequent application to maintain good dps, and whm doesnt have the benefit of shielding. While whm does have more potent heals, medica II generates alot of aggro and uses a lot of mp for a relatively weak heal, Asylum is on a 90s cooldown and only lasts 24s and if the tank has to move it's nullified, but is highly useful when available. Regen is very useful, but doesn't really compare to shielding + fairy heals. Especially considering Regen ticks every 3s, just like the fairy, for half the potency. They do have Divine Seal, but that's only a 30% buff which still doesn't match the fairy, especially a Roused Fairy.
The thing about this though is that Sch is a highly capable main healer, it just functions better an off healer. It can handle AoE increasingly well with Indom and Succor shields+ET Succor. War is also a highly functional MT, but again just better as an OT. But that's another discussion. Though I do somewhat agree that dual sch is a disadvantage, it is still more than capable of any content atm.
I agree that this is the crux of the issue. We can argue back and forth all day about which healer does what better, but at the end of the day sch brings good healing, good dps while healing, AND utility, while whm only brings the first 2, and less of the 2nd imo.