Yes, there is a reason to make block generation cryptologically "expensive". The difficulty level increases with the growth of the network, in addition to keeping new distribution fairly even, to prevent a brute force attack upon the blockchain. The total proof-of-work of the blockchain is massive, and would require a surreal level of computational ability to "fake" prior transactions, even if other safeguards didn't exist. This is the point, as such an attack on the blockchain would undermine the faith in the currency system. Even at this early stage, it would take nation-state level resources to overtake the collective power of the network, but it is still possible if an attacker is willing to commit enough resources to the goal. It would still take much more than the total market value of Bitcoins to succeed at such an attack.
In a presumed future with a market as large as PayPal(TM), the difficulty would presumedly be high enough to render such an attack technicly unfeasable in addition to economicly unfeasable.