Harada says that unlockables were originally implemented in order to extend the life of arcade games. As characters are unlocked, players would have more reason to come back to the machine. We definitely wasted a few quarters just for the chance to play as Kuma, so we'd say the plan worked. Harada said that unlockable characters were added to the home version for the same reason, implying that it would discourage gamers from renting the game instead of buying it.
Fast-forward to Tekken 6, and Harada says that unlockables are "outdated now ... especially with online play." He notes that today's gamers would be irritated by competing online against characters that they have yet to unlock. Thus, you won't find any unlockable characters in Tekken 6.
Of course, history tells us we still might see some characters for sale.