"Psychologically it might be better if users have tokens that can only used for account creation and for nothing else. Although it can be transferred (which enables it to be used like a currency), its primary function is account creation so that's how most people will use it most of the time."
While I do support the sponsorship model for new account creation, the statement I quote here is an unwarranted assumption. For example, people can buy stuff with Air Miles, which were never intended to be a currency with that potential for use outside of airline promotions.
Without deliberate breaking of functionality of any token which is tradable, it is impossible to state categorically that it won't become just another currency.