Changing the content type to include format=flowed means you're not sending plain text anymore. The content is some lightweight markup that's compatible with renderers that only understand plain text. This may seem pedantic, but it leads to real-world usability issues when folks with noncompliant MUAs quote or forward your flowed email; noncompliant meaning compliant with every standard except the one RFC that introduces format=flowed that isn't linked by the other RFCs one follows when writing an MUA, which makes sense since this is a one-off hack introduced by Eudora engineers and haphazardly implemented by some desktop clients.
In short, I completely agree with you.