Added to what Kantay says, anybody rational wanting to run a glass cannon mage on hard difficulty that is not a lone wolf will want to spend points on man-at-arms to at least level 2, but preferably further and best of all to 5 for Weather the Storm, since getting 25% hitpoints (the default from hard + GC) is terrible compared to getting 35%-50% (man-at-arms 2-5 + Picture of Health + hard + GC.)
(The modifiers are additive and applied to the total health from all sources, so a hard mode GC man-at-arms 5 with Picture of Health has exactly twice as many hitpoints as he'd have had without the skill and talent.)
And anybody running a hard mode lone wolf glass cannon, while not being in quite as desperate a need of extra hitpoints due to the lone wolf modifier also applying additively, can surely afford to spare the 15 points necessary to get Weather the Storm.
