With respect, it is some time since I've read the various email RFCs (822/2822/1123 etc.), but I cannot recall any of these email standards saying that emails (a) have to be short and (b) that formatting doesn't matter. In fact, there are numbers of RFC standards that actually specify the standards for text formatting, for example, the Multipurpose Internet Mail Extensions (MIME) standards have allowed 'rich text' formatting in emails for years now.
Thus, to be compliant, email clients should comply with these email standards—this means their editors should also format text to those standards. I contend that there is indisputable evidence that Thunderbird does not fully comply with these internet standards by a considerable margin (nor has it ever done so). Moreover, in parts, Thunderbird is so bad/non-compliant that it is essentially not fit for purpose.
This has absolutely nothing whatsoever to do with the fact that some users will only ever use a subset of those features [standards] when composing emails.
As long as users comply with the email standards, they are entitled to use any formatting they so desire. Whilst many emails are short, the fact is that there are those who often send highly formatted emails of many pages in length.
As fauigerzigerk says, in many instances, attachments are often not suitable so they must be sent in-line. This then requires the editor to handle in-line text/images etc. correctly (Thunderbird does not do this without fault).