Currently, the Tokens of The Twelve needed to buy a (Heroic) True Heart of Wood are BtA. This lets players run content on any character they possess and reap some of the benefits from the time they spent. Now, Tokens of the Twelve, Challenge Mats and Commendations are functionally the same; you collect a number of them in order to get a certain item necessary for your character to progress or become more powerful.

The problem with making such items BtC is that it forces the player to run content on one character and one character only in order to obtain it. This is not a problem if you are lucky enough to run with a static group. If you, like a lot of players, make a group of random players (PuGs) to run a quest or chain of quests, this is a bit more problematic. A pickup group is random, and thus you can't count on a divine (or healer) or a trap-disabling character, or whatever other kind of character you need to show up when you want one to. That is where extra characters on the same account come in. I've run a lot of raids, and sometimes a player will drop group and switch characters if the party is lacking a character that fills a key role.

Now, here is why BtC materials/commendations/etc. make things awkward for a player. Say that a guy, whom I'll call Bob, is playing a melee character in a PUG, and needs one more commendation to obtain the heart he needs to TR. Bob also has a trap-disabler and a divine of similar level as his other characters. However, the group is doing a difficult quest with a lot of traps, and no one is willing to spend their money to buy a gold seal trapper hireling. Now Bob has a decision to make: He can either stay silent and take the risk of failing the quest in order to get his commendation, or he can switch to his trapper and help the group at a cost to himself.

Whatever Bob decides, I don't think he'll have fun doing it. It's one thing to want players to make hard decisions when choosing enhancements, but.... no one likes to have to make this kind of hard decision, devs.