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.
Trump was also the target of "politically motivated judicial scrutiny" (and rightfully so!) So I guess he would be justified in pardoning himself and his entire family, right?