It's a three-star shield so it can't easily be exploited in later depths. It has no continual line which makes it worthless later on unless you're a collector. Even the Shield gives very small boost in health which you think would shine! You're wrong.
So why not give Scarlet Shield a Scarlet "Fortress" to protect allies? This may be useful to new-players or even pros willing to help younger-knights help defend them in their time of need.
Or just give it its own line up to 5 stars
This way people won't feel bad about having wasted CR on a dead end item that gives a small HP boost.
You could use the Swiftstrike Buckler argument but seriously the Scarlet shield's boost only shows when you get hit (which most people avoid doing) while the SS Buckler gives its boost to the 2 most common weapon types in the game (sword and shield).
Even a bomber would rather have a Jelly/Skelly/owlite shield over a scarlet shield.
If the 5 star version gave more health, I'd use it very often. I like being a tank.
But for the 3 star fortress, I honestly don't know if it would help. Most people are very slow to realized when a chance of safety is given to them I have never seen another person try to hide in a Nitrodome or even catch a ride on a Sera Aura. I don't think the Scarlet Shield Fortress will be any different. People want to kill, not live.
Which is why the Swiftstrike exists!
I'd like to see shields spiced up a bit– some special abilities specific to certain shields. You look at the wiki now, and the only special effects now belong to the swiftstrike, the thorn shields, and the tortoshields. Whereas every other piece of equipment has some sort of flashy power, shields are still rather utilitarian, and it'd be really cool to have shields with stuff like a "Scarlet Fortress" or advanced bashing properties.
The Scarlet shield is currently the only shield to affect a player's HP, albeit a rather pathetic boost of +2 HP but it is the only shield that does so.
/yes