Fortitude is useful but Will needs to be prioritized above it.
Death Ward (or late game legacy) renders you completely immune to death effects which is literally covers almost everything fort can do to end your life. Flesh to Stone is one of the very few effects that would say otherwise. Fortunately not even a Solar prepares it on their default spell list. What else? Disease? Lame. Poison? There is immunities to that. Sickened? Nice debuff, intimidate is better, but you get better results from will based CC spells. Heck, Flesh to Stone / Baleful Polymorph are not exactly super great given the touch range.
Will on the other hand, even with Protection from Evil, Mindblank, & Death Ward protecting you from mental domination moral changes and flat out death you are still subject to things like Slow. So add on Freedom of Movement if you will. You've got four spells running here to Fort's one and you still have exceptions like teleport and planesshift(to hell) which are the same as fort's options to deal with. Even giving up one of those can lead to rape, example lose PoE and now you're dominated and killing your teammates. Antimagic Ray and Disjunction will gladly remove some magical effects, Maw of Chaos (yes it is a will save partial) still deals more than unoptimized melee and even Backbiter can murder a strike using material adept at low levels. Those are options fort doesn't have and the list keeps going.