Why is Modalism so bad?

I want to first get it clear that I'm not a Modalist nor promoting Modalism. I am just wondering why is the idea that God is a single being who reveals himself in three different 'modes' (Father, Son and Holy Ghost) considered such a grave error. Why is it so essential to believe that God is three distinct persons? To the point that Modalists are often not even considered Christian, and their baptism is believed to be invalid?

I believe that a lot of regular lay Christians are Modalists in good faith. Many probably can't even see the difference between Modalism and orthodox Trinitarian theology. At least I couldn't when I first learned about the Trinity as a kid.

Can someone explain to me, why is Modalism such a grave error?