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.
That reminds me of the early 2000s, where there were a lot of US debates around around terrorism and "harsh interrogations" i.e. torture.
A certain bloc of politicians and commentators kept bringing up a hypothetical scenario where there was a nuclear bomb counting down, and some guy wouldn't admit where it was hidden in a major city. My favorite response to that involved presidential pardons, something along the lines of:
1. "So what? If everything you say is true, then the authorities would simply torture the guy and seek a pardon afterwards. We already have an exceptional mechanism for those exceptional situations, meaning that's not a reason to change it."
2. "Conversely, any interrogator who isn't confident of a pardon is on who does not believe it's at ticking-bomb situation, meaning they cannot justify torturing someone anyway, they just want to do it to make their job marginally easier. That's bad, so it should stay illegal."