(I work at Inbox.)

    IMAP libraries are good enough to not have to pay for 
    a wrapper when using most email providers

I found this actually not to be true when building my own stuff. Although IMAP is a "standard," the various different extensions and quirks mean you need to essentially build for each provider initially.

There's also a lot of stuff in Inbox API does that's not available in IMAP. Like the files endpoint[0], or threads as a first-class object[1].

Plus, you don't need to worry about MIME or character encodings with Inbox. The reason we don't great email tools is that they're hard to build.

[0] https://www.inboxapp.com/docs/api#files

[1] https://www.inboxapp.com/docs/api#threads