We disagree on the scope of limitations available. The type of situation in which I imagine I might allow one of my players to use an Item of Power like the one we're discussing is one where the patron who gave the Item to the character exercises close control over the situations it's used in. The following quote gives clear reasons why what you referred to earlier as "unimportant backstory" can matter quite a bit.
The thing is, the rules of an IoP work through Compels. Having a sponsor breathing down your neck is, from an OoC perspective, not a weakness. (It is one IC, of course.)
This is important. Because if you don't take it into account, then you punish interesting character concepts.
Also, I used the prefix "mostly" for a reason, please don't remove it.
I'm still not saying that I would allow a -1 Mythic Toughness coat in my game. I'm just saying allowing it to be used when there are no strings attached to it is so far out of the question as to be laughable.
EDIT: It's no less laughable with the strings.
First, you are obviously not a powergamer, not by my definition.
With respect, I don't care about your definition. Any three gamers have four definitions of powergamer, I refuse to care about them.
You are arguing against an Item of Power that could give you Mythic Toughness for one refresh (though you're arguing against the reasons I gave when supporting your argument against it...), which is a very efficient way to spend that one refresh, whereas a powergamer would be arguing that it's totally rules legal and therefore, there is no reason he shouldn't have it.
It is totally rules legal, and by the rules there's no reason someone shouldn't have one. In fact, the rules support it much better than they do an interesting IoP.
But I like rules, and I value good rules, so I care about the fact that it shouldn't be totally rules legal.
Plus, I figured the OP deserved a warning. I'm not sure how crunch-savvy he is.
When I use the word "powergaming", I don't merely mean normal character optimization such as efficient refresh spending... that's just "gaming" to me. For a better understanding of what I mean by "powergamer", Google Pun-Pun. It's a D&D character concept that allows a character setup a certain way to be as powerful as the player decides he wants him to be. Like all stats in the tens or hundreds of thousands and every spell and feat and ability in the game. That, to me is a powergamer's win condition: a way for him to be able to beat anyone in a fight and overcome any challenge.
I'm aware of Pun-Pun. In my opinion, he's a work of art.
My earlier comment about needing a reason beyond wanting the power and being able to afford it in order to purchase a power was not supporting some lofty ideal of 10 page character concepts and having written a biography about your character, it was just a statement about the nature of this type of game. I have a player who is currently playing a werebear. He has four available refresh. If he comes to me tomorrow and asks to buy Evocation, he's going to get told, "No, werebears are not wizards and don't get evocation." If he comes to me tomorrow and asks to upgrade his inhuman toughness to mythic toughness, he's going to get told, "No, you transform into a normal bear, not one covered in dragon scales.... you're only inhumanly tough." We also have a wizard with two available refresh. If he asks to buy inhuman toughness tomorrow, he's going to get told, "No, you're a mortal wizard and are pretty definitively humanly tough." That being said, my players wouldn't do that because they understand their character concepts. If the abilities don't fit the character concept, they're off limits.
Ensuring that abilities fit concepts is good practice. However, it has nothing to do with game balance.
Very powerful characters can and often do have good backstories. Look at Elena Blackcloak, or The Twice Betrayer Of Shar.
So while I totally support your efforts to keep concepts coherent, I must stress that you aren't keeping the game balanced by doing so. You're keeping it sane, but not necessarily balanced.
PS: Your werebear would become weaker if he took Evocation. By preventing him, you are not keeping him from being able to defeat everything. You're keeping him from gimping himself.
PPS: Were-forms don't actually turn into animals, IIRC. They turn into their mental images of animals. It's a pretty good justification for excessive Powers.