That's like complaining that your web browser gets full access to your browsing history, or that your IM client gets full access to your plain text conversations, even when using E2E encryption no less!

No, it's like complaining your IM gets access to your e-mail conversations. Or browser gets full access to your phone calls history.

Access to your e-mail account allows you to reset password in most services associated with it.

No, it's a client that uses your email account for purposes involving reading and sending email. Exactly like Thunderbird, or Evolution Mail, or Outlook, or Android Mail, or iOS Mail or countless other clients that have full access to your email client by design, and without issue.

People generally take issue with handing over access to their email account to third party services. People don't generally take issue with a client under your control accessing an account under your control.

OK, fair enough, depends who provides the app. So far the project author(s) seem anonymous.

(And no, the fact of being open source is not enough, at least not until certain popularity is reached)

I've tried many XMPP clients on mobile but I would be afraid to test many e-mail clients.

We are http://www.open-xchange.com, http://www.dovecot.org plus people from the Delta Chat community and http://spikenow.com - plus whoever whats to join.