Not sure why you'd mock this use of Emacs, after all mails are text and as such very well suited to being composed with a text editor...
The point is, you don't have to have embed your MUA inside of your text editor in order to use your editor for composing mail. There's nothing particularly wrong with that approach, but it's not the only way to get that benefit.
Because it's all inside Emacs, text from other contexts -- code, shells, mailboxes, and compiler and tool output -- can be accurately and quickly pulled into the messages you compose, improving the content and reliability of what you're saying in mail.
For example, if I want to use a complex identifier from a source file I've just looked at, I can autocomplete it (M-. or find-tag) quickly from a substring. I can choose autocompletion suggestions simultaneously from all contexts.
Obviously, copying and pasting regions from those other contexts is much more immediate too.
This is in addition to the advantages of having a familiar and powerful editing tool for formatting, tidying up messages, trimming unnecessary content, etc.
I could, but wouldn't want to, live without it.
You are right, emails are texts. Managing emails and contacts and calendars can go (and usually goes) well beyond text editing, though.
I got that, sorry if my reply wasn't clear. When I said "not sure why you would mock it", what I actually meant was "why one would mock it".
Anyway, as I've argued before [1] the whole Vim-vs-Emacs (and Emacs-vs-Vim) fanboyism is sad and a waste of many talented people's time. Can't we just get along and instead focus our efforts on converting the legions of people who understand neither Vim or Emacs and embark on writing the n-thousandth Notepad clone? :-)
Vim user here. I use mutt for email (which opens vim for editing emails), Vimium for Chromium, and I've set vim to be my default editor, my shell accepts vim commands, and my tiling window manager uses vim-style keybindings.
So, I almost never have to do anything in an environment where I don't have access to all of the power of Vim... and yet my text editor remains simply that - a text editor.
No problems with multitasking, because I've never needed to do more than one thing at a time in Vim, and I can't imagine ever needing to. All of those things that you're thinking of as separate processes in Emacs are things that I delegate to dedicated programs that specialize in doing those things. But I can get all of that modularity without sacrificing any of the vim key integration.
Not trying to sell you on using Vim, but just trying to explain the different reasoning.