There are programs out there they will let you use your own POP3/SMTP clients to send and receive e-mail via Yahoo. Since they aren't
technically permissable, I will leave it as an excercise for you to find them.
<nasty BOFH rant>
Not everyone employs the use of HTML e-mail. I, for example. won't read e-mail if it is in HTML format. My thinking is, if I wanted to see a webpage, I would load my web browser.
That being said, you must remember that not everyone feels the same as you regarding IncrediMail. Not only that, but a lot of commercial ISPs run anti-spam software that IncrediMail triggers due to its high HTML content.
Not everyone views pictures in their e-mail, and if they do not, the recipient will see nothing but URLs crowding the e-mail text (Outlook XP does this with a registry change).
Speaking as a sysadmin, IncrediMail increases the size of e-mail almost ten fold, creating more bandwidth, less storage space on servers, which results in higher costs for everyone.
Don't use it if you can help it.
</nasty BOFH rant>