Emails which are written with HTML are annoying.
They look bad with some clients with plaintext inlines (at least on Icedove and Thunderbird).
They add empty lines to emails. Have you ever tried looking at source of one?
They can include very annoying formatting.
Do you like reading email which is for example, all in bold, cursive and with some notice attracting colour, like red? And what if it's even written with some very annoying font?
They don't show correctly in some emails clients.
At least guidelines of Debian mailing lists, Ubuntu mailing lists and Linux Kernel Developer mailing lists forbid writing emails in HTML. Hopefully others do to, at least they should.
PGP/INLINE signed or encrypted blocks get invalid, when HTML adds spaces to them. This seems to be very popular issue (probably, because K9 mail has interesting default settings of sending HTML and topposting...)
Are you sure? Debian wiki has instructions how to write plain text with many clients.
Are you sure? Debian wiki, which was mentioned eaerier also includes instructions for many webmails about writing in plaintext.