I suspect that the idea originally was to give some safety valve but if it is used more than a few times by a President, it makes a mockery of it and it should be removed as a power. How can a President ever decide that the entire legal process is flawed and their opinion is right? If the sentence was too long then change the sentencing guidelines.
On its own it is not that bad an idea for someone who carries a mandate of the majority of the population to be able to grant pardons.
Would it be a better system if the not-allowed group is totally dependent on the people that are allowed to vote?