Thank you so much for your reply. Yes I understand the logic, but as you said, you know what's my main point. steem-ua.com is an example for the workaround. If you log in steem-ua.com with a posting key, then you can log in dpoll and use it! for instance. This means that users should be able to log in dpoll with posting key (if they already authorized the posting right, of course). This is the main point.
In any case, there should be an easy way for apps to allow users to log in with posting key afterward when the session is expired, or with new browsers or whatever. Hope SC3 will resolve this problem. Thanks again!