Most mail recipients I deal with don't mind HTML email.  And HTML email
provides more flexibile formatting etc.  So I set it on by default.

Some recipients however object to HTML formatted mail.  Usually the hard
core geeks who use mutt or other text based MUAs.  Sometimes you also don't
want to post HTML to mailing lists.

Having to remember to turn HTML off is annoying.  Most of the time I forget.  

I know there is the "Wants to receive HTML mail" option in the contacts
management.  However it defaults to off.  Setting this to on for every
contact that I want to send HTML mail too would be downright annoying.  

What would be better would be a select box that had three options.  "Use
Composer Default", "Only HTML Mail", "No HTML Mail" with the first option
being the default.  That way for those who object to HTML (and mailing
lists) you can set it to No HTML Mail.  

Then when you compose a message the mail composer can check the recipents
against the address book and switch HTML on or off accordingly.

If this feature could get into the next version that would be great.