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).

And printing is about the most horrible thing ever invented. I only do it under threat of financial loss (return labels, letters to tax authorities). There's nothing as broken as printing. It never works flawlessly and I can't focus on printed text anyway.

Printed text has no search, no copy and paste, no changing font sizes or brightness, no hands free reading, no place to put paper without it getting fatty or full of coffee stains.

So I guess the conclusion has to be that there is no general right or wrong when it comes to the length of text in email. I depends on who you are sending it to. I pity whoever has to correspond with the both of us :-)