Bill Zebub
“It’s probably Matt Mercer’s fault.”
The main argument for floating ASI is largely the same one in favor of racial ASI. IF* it is the case that racial ASI "helps the themes, motifs, archetypes, culture and worldbuilding of the game" by "attaching the fluff to the mechanics" --if that's truly the case, then if you want a world with different or merely expanded themes, motifs, archetypes, culture, and worldbuilding, the existing racial ASI will either be a limitation or irrelevant. So we can ask whether the implicit setting of the core books should hew to those classic archetypes. If the answer is yes, proceed with racial ASI, if the answer is no, then you need a more versatile mechanic.
[*if racial ASI serves that purpose. The other argument, one more against racial ASI than in favor of floating ASI, is that racial ASI doesn't actually do a good job of reinforcing those classic themes and archetypes. If you can say to the powergamer, "the +1 in your stat doesn't really matter that much for how your character feels at the table, it's just 5%," then that same argument can be used to say, "the +1 doesn't really help distinguish races from one other, it's just 5%." Further, the fact that ASI is not unique to race but also comes from class progression means weakens its distinctiveness. Here's where we can turn to other games and suggest racial feats and abilities as a better mechanic than racial ASI for the purpose of developing themes, motifs, archetypes, and cultures. Alternatively, we can turn to OD&D, where there were no ability score modifications but races were restricted to certain classes, or basic, where race was class, and see if the mechanics there do a better job than 5e.]
To give you a sense of my stakes in this discussion, if I wanted a game that supported the classic themes, motifs, archetypes, culture and worldbuilding of dnd, I would not choose 5e. There's too much in the game that doesn't work well toward that purpose. Instead, I would choose a version of basic dnd (probably Old School Essentials). But I can take at face value that for other people the default mechanics of 5e do a good enough job of providing the necessary archetypes via racial ASI. Again, if that is truly the case, then for those wanting a slightly expanded array of archetypes need to add versatility with the mechanics.
In general I’m wary of everybody just Liking every post on their own side of the debate, but I think this post lays out some of the thinking especially succinctly and clearly.