Warning: Contains spoilers for Harry Potter and the Deathly Hallows
In Harry Potter and the Deathly Hallows Harry, Ron, and Hermione take polyjuice potion to turn into three seemingly random Ministry of Magic employees. I have no doubt that their choices were fairly random based on who entered the building by that specific door at a certain time, but I think Rowling had a lot better idea of what she was doing when she chose Mafalda Hopkirk for Hermione and Reg Cattermole for Ron to impersonate.
By impersonating Mafalda Hopkirk Hermione got a front row seat at the trials of muggle-born witches and wizards. She got to see what would have been her future if she had not already been a wanted criminal for helping out ‘Undesirable No. 1’. She remembers getting her Hogwarts letter and discovering she was a witch and buying her wand just like the muggle-borns on trial did. She can relate to the horror and fear that they are feeling because she feels it as well as she watches the dementors hover threateningly nearby.
As Reg Cattermole Ron got to experience what it would be like to be married to a muggle-born in that dark new world. It’s safe to say that by this point in the series Ron has figured out that he has feelings for Hermione. His family have always been ‘blood traitors’, but his personal connection to Hermione makes the idea of the Muggle-Born Registration Commission even more disgusting to him, if that is possible. As Cattermole he had to run around the building trying to stop the rain in Yaxley’s office in the small hope that his “wife’s” sentence would be less. In this chapter Ron gets a glimpse of what his and Hermione’s life could have been under different circumstances if Voldemort was never defeated.
If Harry had gotten the identity of Mafalda Hopkirk or Reg Cattermole that chapter would have been very different. Yes, he was just as disgusted by what was going on in the Ministry of Magic. He had a personal connection to muggle-borns himself, but it would not have had the same effect on him as it did on Ron and Hermione. By taking over those identities they got a look at what their own future could have been.