Update:
I did not expect that the disadvantages of plain text e-mails ca not be seen at a glance. However, here the points you can detect from the images:

• you can not tell which part of the e-mail is the signature (often filled with its bland legal texts) and which is the important part
• the e-mail takes nearly twice the space of an HTML e-mail and still does not fit into that window
• there are no clickable links for URLs, Skype addresses etc.
• the company logo is missing

Still, there are much more disadvantages which I do not want to mention here. The worst situation is when you are having a long conversation with sombody using HTML e-mails and then someone joins with his plain text e-mail client and destroys your e-mail. All that remains is a fragmented text with lots of unnecessary white spaces and line brakes, e-mail body and signature mixed up and in most cases all your umlauts (ä, ü, ö) or other characters are replaced by some strange ASCII characters.