After doing some thinking, and taking peachykeen's point into more consideration, I will concede and give it some due credit.
Yes, the current method implemented by MCP does add a useful mechanic, buffering health before a battle, and increasing the effectiveness and allowing the use of more powerful healing spells.
But this mechanic requires longer durations to be useful.
I absolutely agree there.
With long durations, more than the usual for a single fight (multiple fights you can chain the spell or have a high power version) it would become far more useful than it is. In fact, to build on Gaius Atrius' comment:
It simply must be given some sort of failsafe...
How does a fade-out sound?
For example, the player casts a fortify health spell, boosting their health by 50 points. When the spell wears off, instead of them losing 50 points from what they have remaining, the spell slowly fades. Let's say they instead lose 10 per second, up to the amount given by the spell; or they lose the health over the course of 5 seconds (20% of the boost each second).
That would give players a warning and a chance to heal instead of dropping dead out of nowhere (I understand that it's easy to lose track of spell durations in a big fight), but it doesn't ditch the strategic nature of the effect. It seems like a nice balance, removing the major problem you're worried about and keeping the aspects I appreciate. Making the spell somewhat cheaper and allowing longer durations would also be beneficial, together they would make for a very powerful (but not risk-free, and so not unbalancing) spell effect.
Sorry if in my previous posts, I seemed abrasive and unassailable. My thought process is humble, but my rhetoric can be very abusive at times. I have considered your points of view however, know that.
And the same from myself.