the point is not to be able to "happily keep separate email identities completely separate". It is to have a client that knows by ITSELF which one it should switch to in any moment depending, for example, on who you write or reply to.
In Thunderbird, for example, (and Kmail had the same limit, last time I checked, see http://www.makeuseof.com/tag/how-to-set-up-mozilla-thunderbird-3-multiple-email/)
"When you > Write an email, you???ll see a drop-down option under > From:. This is where you can select any of your email accounts."
How boring. With Mutt profiles, instead, you don't have to do that every time. You configure it once per every address you want to send email to, it remembers what to do.
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