OK, so I'm glad that you read my post as adding a aesthetic evaluation of the situation rather than a game balance one. However, I don't think that's necessarily not a "valid argument"; if we threw out aesthetics, we could easily (and much more cheaply) play the game with bottle tops or chits. For the record, I'm not saying I think anything needs to be changed, just suggesting that there are more than game balance issues that we might throw in the mix.
I do think that Wyrd does use some rules to enforce theme. Off the top of my head, for example, Lazarus' "hate the maker" rule. Given his fluff, it would make no sense for Lazarus to be in a Guild crew, as his rebellion is from the Guild is intrinsic to his story, though I can't think of a game balance reason that he couldn't run with Guild. What I am saying is I think rats are closer to the Lazarus rule than to your examples, meaning more central to Hamlin's identity than other masters.
Again, I'm not saying what I'm offering is a "strong argument" or that I'm advocating a change. I'm just offering a fluff/thematic consideration. I just think it's weird and more jarring seeing rats in a non-Hamlin crew.