For one, it discourages polarization. Yeah, I'm sure many in California are happy it's a near 100% Blue State due to the coastal cities aside from San Diego, but that has consequences in being taken for granted or worse.

And anyway, speaking as someone who lives in a true 100% Red State part of Missouri (Jasper County in the extreme SW), that just changes which election matters, the primaries are were the action is. E.g. this year every county position I could vote for, plus my state house rep, was unopposed a couple of days ago. But that was most certainly not true in our August state primary!