My 2 cents as L100 sage which is on pure support mode..
Also, I think Sedy adjusted a few of the sage skills since Ash wrote this guide, so not everything is relevant anymore.

1) I found the DoT skills mainly useful for flag interrupting in FM
2) Totally agree on the Focused Mind skill - essential if you are running dungeons because prophets get a lot of aggro from all their long duration heals. However, I find Elemental Exposure fun to have because it gives your magic dps a boost if you have less sorcs in party (wiz don't have this debuff)
3) At end game, you should expect to have instant cast on this skill - because of the short cooldown, you will end up spamming this heal a lot so its nice to max. Agree that the effect from the Def reduces.. but better than nothing~
4) Delayed cure... I guess it depends on timing a bit. If you are busy healing a tank, there are often 'lulls' where the hp is full.. then you dump a delayed cure and celestial sphere so later when its needed you can get those extra heals. Also helpful for PVP if your teammates like to run away from you.
5) Sedy adjusted this skill which is why the description changed
6) can never have enough -ct
7) Well, as sage you will be expected to have this available to buff your party

[Necroposter mode on]
A little IMHO 'bout sage/prophet's skills.
1. I played both Warlock and Sage and i find the shaman's fire element attacks useless foe sage. I use [Arcane Curse] and [Flash] in my skill rotations instead. I dropped [Arcane Curse] at lvl 45, when i got [Judgement Arrow]. [Flash II] was still usefull at lvl 65 when i stopped playing sage on my previous server. Dunno 'bout end-game, but i really think that an instant-cast skill with short cooldown is usefull everywhere.
2. You forgot to mention a great shaman buff - [Focused Mind]. It gives -6% MP consumption and - 40% aggro gain. Stacked with the Prophet's passive skill [Soothing presence] one'll have -70% aggro (ang there's also jewelry with the same effect, so we can reduce aggro gain to an unnoticeable amount. I wonder if one could reduse it up to 0%? ). I took [Focused Mind] instead of [Elemental Exposure] - those who can find it usefull have it themselves.
3.[Forcefull Rescue]'s +2500 Def is a WOW up untill lvl 50+, where the wow-factor decreases and it won't play it's former role in boosting survivability. I dropped it to lvl 1.
4. [Delayed Cure] stops being a lifesaver at lvl 45, where most jobs which could use it receive their own built-in healing skill. So i keep it at lvl 5 as a prereq to [Flash I].
5. [Fortified Armor] gives only HP and HP-regen, not damage absorb. But still, i find about +5000 HP quite usefull.
6. [Empowered Cloth Armor] is a must. Combined with the shaman's [Spiritual Blessing] and bonuses from costumes will reduce your casting time by a noticeable amount.
7. I have doubts about the usefullness of [Potential Growth] at levels past 85, but if you think it's really usefull - i'll keep that in mind.
8. Have nothing to mention about the rest of the guide.... yet

[Necroposter mode off]
PS Just to add some visual information - the shaman and sage skillbuilds i used. There were no prophets at that time, so got no opinion 'bout their skills yet.