I think it's pretty easy to make poor use of HTTP to the same end. Imagine you're dumping traffic between a client and a server and the exchange is:

   GET /asdfasdfasdfdsaf
   X-Asdf-Asdf: 83e7234
   
   HTTP/1.0 202
   X-83e7233: 1
   X-83f730b: 4
   

It's text, but you still have no idea what's going on.

Overall, I think it's kind of a wash. It's basically equally easy to take a documented text or binary protocol and write a working parser. Neither format solves any intrinsic problems -- malicious input has to be accounted for, you have to write fuzz tests, you have to deal with broken producers. It's a wash.

People like text because they can type it into telnet and see something work, which is kind of cool, but probably not a productive use of time. I can type HTTP messages, but use curl anyway. (SMTP was always my favorite. "HELO there / MAIL FROM foo / RCPT TO bar / email body / ." Felt like a real conversation going on. Still not sure how to send an email that consists of a single dot on its own line though.)