Stoneskin becomes even more useful when you:
a) upgrade your armor
b) get Protect and Shell
c) use a shield. (a partially blocked hit will not take as much away from your Stoneskin protection)
From my experience, all those defensive measures are considered before damage is applied against Stoneskin.
I wrote a blog post on the topic of Conjurer buffs: The Importance of Being Buff(ed).
Bottom line is, as a Conjurer, if you're not taking advantage of Stoneskin, you're selling yourself short, in my opinion. I don't often cast it in solo battles, but as I go into them... saves me from casting a Cure II during the fight, and I like.