I think his point was both Swiftcast and Stoneskin are useful tools. It's not a case of why use Stoneskin, it's why not?
Nope, I said he's already shielded. There will be times when your tank's full and/or shielded, and you will be unable to contribute simply because you don't have stoneskin.
You're going to run into situations where the other healer is busy, and you're not, and if you don't have stoneskin you just have one less option you could have used to help the team. Of course I used extreme examples, as it's where it's usefulness is most obvious. In your average raid it'll make the run smoother, but won't literally save someone from dying every time you cast it. If it were that good it'd be horribly unbalanced.
Let's put it this way: compare you to another sch. Everything's the same, gear, skill, experience, except he has stoneskin and you don't. He's got more tools, and will have the advantage. Same argument I said to Gilcris below: you can literally just cast only psyhick adlo and lustrate for any content currently in the game and still clear it. You'd be making life harder for the other healer but it's still doable. The point really is: you have the tools, why not use them? If the answer is you can't be bothered to level cnj then the answer is you're just lazy. It's not a bad thing, this is still just a game we play for fun. If you don't want to get it, don't. But the fact remains, you've not got all your tools yet. By simple comparison, you're not as good as the sch who does.
Thought I'd try to address this too. The nature of "necessary" depends greatly on the group you're with. The most basic tools a healer has access to is sufficient to pass even the most difficult content if the group is efficient: No one's getting hit by avoidable damage, the tanks are using cooldowns at the right times.
It's when you're in a difficult encounter, either because the encounter is new to your group, or the group itself is new to you (hello PF DF), is when you want to be prepared for anything.
The example I quoted with Ifrit EX is a real use for stoneskin. During the third nail phase triple incinerates will occur, and the entire party is taking damage due to the nails blowing up. If stuff is going slightly badly, and your other healer is preoccupied with healing the team, your tank's already shielded.. you have no stoneskin to help reduce the damage your tank's about to take with the triple incinerate. Worse still if the tanks messed up the swap and he's got stacks already. Is it "necessary" then? I would argue yes. Is it an extreme example? Yes, but I've personally gone through this, and I could see it happening quite easily again.
Tank dying at that point can quite easily be the difference between success or failure.