It's really unlike any other skill and being able to raise it is unrealistic. Practice can improve your aim, or your dodging, or any other skill, but no amount of training is going to let you get shot 40 times.
Yet nearly all Role Playing Games give significantly more health/HP to more developed characters to that those characters can face tougher (unrealistic) obstacles. Bears, Dragons, Super Heroes, Demi-Gods ... what ever your preferred game is, it's there. A player, irregardless of their level, should not be able to survive 8 shots from buntline either but that's the "skill/ability" that is placed in the game for higher level and advanced characters to complete/survive/thrive against the more advanced weapons not only in this game, but in any RPG. I think something should be done to discourage the rampant placement of a pure health character but not by nerfing them and bringing other players (who have not skilled in health) up to their level.
I think these kind of proposals, not just in health modification but all proposals related to fort fighting, show the deep passion that the fort fighters have for this game yet at the same time it also demonstrates the frustration they are having with this aspect of the game becoming "stale" (for lack of a better word because it hasn't descending into boredom yet but it's headed there). With all fort being of the same design, except for the three sizes, the battlefield is nearly always the same, there is little change in strategies which are used and fort fighting is more like a dance than a combat competition. If Inno wants to invigorate the entire fort fighting concept of the game then they have dozens of suggestions in their forums to add to this feature but I don't see changing the health skill being any type of a fix for the fort fighting problems we're experiencing.
Players make a choice on what type of player they want to be and fortunately there is a feature in the game that allows players to reskill if they want to do so. Those players that want to be "tanks" have made an active choice to do so, just as the dueler as decided to skill for dueling and the quester has decide to spread their skills to maximize their ability to do most jobs. With the reskill potions in the Bonds Shop available, it makes reskilling much easier than it was before as well so if a fort fight wants to do something else they can accumulate the necessary reskill potions or LP buffs and get it done (when they're done with whatever task they've chosen then they can return to fort fighting).
I think something needs to be done to discourage the HP over allocation but not to limit or cap it (and regardless of whether you agree with the term CAP or not, effectively that is what it is when you can't add your SP's to health anymore then it's CAPPED ... even if it's CAPPED by level, it's still CAPPED). I've often thought "Why are the other five skills set to ^.4 but not health", then I ran the numbers and found that health would need to be somewhere between ^.7 and ^.9 to make it playable there, but that's just one idea on how health allocations could be given "diminishing returns" but it would not stop players from allocating all of their SP's in health as they do now anyway.
Personally, I love Fort Fighting but occasionally I need to break from the insanity that accompanies this feature of the game (telegrams, spam whispers, absolutely vile and venomous chat ... it all just becomes too much after a while and I have to step back for a while). A bit off topic, but there are many more problems with fort fighting than rampant health abuse. Find out what causes players to leave fort fighting and then you have found the areas that need to be addressed instead of curbing runaway health.
The argument in this proposal is to return the SP for health to the players so that they can do other things in the game. The players have the choice to do that now and they have chosen, for better or worse, to dedicate their skills to the health skill and become the best fort fighter they can be and that's their choice. These players may not have as much or even any interest in other aspects of the game and if they do then they're free to pursue those other aspects any time they wish to do so by reskilling.