1) Stoneskin costs a little more then a cure and is a slower cast. Your cure has a lot of help to scale and as result restores A LOT of HP, but Stoneskin will only do a flat % of a target's total HPs. Stoneskin draws very little aggro to cast and gives a decent buffer before healing from fight to fight, but casting on targets during a fight is entirely situational. The WHM will have to "get a feel" for what drains on MP they can tolerate and how often it seems worth it. Good to cast on pre-pulls and as you get good, you can refresh it just as a fights ending, any more then that though is what you feel is worth the time and energy.

2) Don't have it myself, but it only has a chance to actually do anything when its cast on something. Anything you cannot count on time and time again IMO is not worth stopping a career path for. That said I may eventually get it as options are always nice, but I won't be halting development just to get it.