You’d think that Google, a company that’s run by nerds, would appreciate that plain-text emails should have monospaced fonts and a limited line width.
Instead plain text emails look the same as rich format emails in Gmail. So when you write an email like this in plain-text in a way meant to convey a quote:

And resize the window:

It gets all messed up.
I think it’s because they know most of their customers don’t do this much, if at all. But that’s the kind of thing that leaves room for things like Letters.app.
I want to think it’s because they don’t want to confuse people, but this is the same company that has tons and tons of Labs features.