I hadn't planned on a bunch of extra dialogue in the first panel, but it occurred to me that Tedd's statement about technical difficulty might not make sense without prior experience trying to work with such things (it only makes sense to me because I've played around with modifying Bethesda RPGs).
It would be different if just turning them into a humanoid turtle creature, but a turtle "form" in which they're still recognizable as the character they started as and such? That gets tricky.
Versatility at a cost yay
This might shock you, but I like versatility. I like abilities with multiple uses, I like cards in card games that give you options, I'd like a transformation watch, etc.
As such, abilities that simultaneously give significant buffs and debuffs appeal to me so long as there is some sort of way to use them.
In the case of this spell, I could see someone casting it on themselves in a "I'm going to get hit either way" scenario, or on a fast enemy they need to get away from.
Again, other schools of magic would probably have a better way to slow, and a better way to buff defense, but probably not both simultaneously. Granted, both simultaneously might not be desired, but I imagine the other two in separate skill trees while ye olde turtle shell spell is, well, one thing in one skill tree.