Then every merchant will have to update clients whenever users want to use a new transaction type. With OP_EVAL, a merchant could keep running the same version for years (assuming there are no bugs) and still send transactions using the latest scripts.

It also improves decentralization, since there will be no need to coordinate assignment of address version numbers. (And there are only 256 address version numbers to assign -- too few to cover all scripts.)

People who request an OP_EVAL transaction will know exactly how many extra bytes it will take to redeem the transaction. They shouldn't be surprised by fees.