Hello,
I much appreciate you taking the cautious side and I urge everyone to do the same and not blindly provide the active key (password) to any webpage (including streemian.com, which I am the CEO of).
The problem currently is that there is no way to have steemit.com update your account properly in order to allow streemian.com to vote on your behalf. The blockchain needs to be instructed to let streemian do so, and to instruct the blockchain, the active key is required.
However, some measure have been take to make live easier for the user such that users can instruct the blockchain properly by "just using" that page.
The security model for the end user is that the page's sources are hosted on a public server (github) and the source code can be easily review by developers (I urge as many developers as there are to take a look and verify the code). I also encourage every users to read the technical details on the page.
Hopefully, we will see a more closely integration with steemit.com to make live easy and secure for customers.
//edit: To make it clear: Streemian.com does not store your active key. In fact it doesn't even see your active private key. The key never leaves your browser and all the modification instructions are generated and signed in your browser!
Thanks a lot for the explanation @xeroc :-) and you are right, I should read the technical details on the page ;-)
Danke nochmal.
Cheers
//edit: Hoping this will come soon