I think this result needs more investigation. We need to know why people open plain email more than those fancy designed ones, rather than just having those numbers.
Designed email, for a lot of companies, is a matter of style, a VI (Visual Identity) for the receiver to remember. It's a good tool to differentiate your services from others.
Also, a well-designed email could increase the efficiency of email reading as you may already familiar with some email and their layouts, so you can guess it's content even you didn't actually read any word in it (Those "What's new" email from Twitter and Facebook for example).
If everybody start to send plain email today, then there will be a whole lot more of reading for the receivers to do.
So for me, I don't reject designed emails, as long as most content in the mail is what I needed. Maybe it's a notification, verification or something like that.
I hate some company send emails which heavily polluted with contents that have nothing to do with me, and if that email was also designed, I hate it even more.