OP is arguing that SS is more useful compared to other WHM heals as the tank gears up and increases health pool. This is only true for the scenario of a poorly geared healer and a well geared tank. Because the same can be said for healers - as they get better gear, their base heals also increase. In other words:

Stoneskin scales with tank Vit/gear
Cure I/II/Regen scales with healer Mind/Gear

In the end, SS is still less mana efficient than Cure I/Regen and has a sufficiently long cast time to not be reliable in movement intense fights. Furthermore, a stoneskin/regen combo has a higher likelihood of regen ticks overhealing as opposed to a Cure I/Regen combo that the healer has more control over. And finally, there is no way to know how much of the current stoneskin has been 'used up' vs remaining. Because of its long cast time, it would be necessary to precast stoneskin as the previous SS is wearing off.