Although the "mirroring" name suggests showing the very same image on both displays, there is little need to see the same view on the streaming device's smaller screen, too. If we could turn the smaller mobile screen off during MHL connections, and maintain the device control through this small display surface as a multi-touch toucpad, such implementation would have the following benefits:
1) The mobile device's irreplaceble battery doesn't drain as fast. Passive connection MHL cables can work with better duration performance, too. The screen is the most power consuming part of a mobile device.
2) Requiring less recharge cycles will extend the life expectancy of the mobile device's battery. Considering the battery's irreplaceable nature in majority of the SONY units, this reduced recharge cycles simultanously improves the life expentancy of the mobile device itself.
3) USB 3.0 based power feed connection on an active connection MHL cable could become less important. Reducing the need for higher power feed, could also eliminate the need for carrying the charging adapter to remote display locations.
4) Turnning off the screen LED, but keeping the touch sensors on it active can convert the mobile device's screen to a decent size touchpad for controlling the device with multi-touch gestures.
Here is how everything could work in my vision for the best user experience:
When the MHL cable is connected to an external display, the physical button for camera on XPERIA devices could automatically function to turn on/off the device display without effecting what is being streamed to external display output. There should also be a setting under Settings-->XPERIA Connectivity-->Screen Mirroring with a checkbox, which converts the turned off screen to function as a multi-touch toucpad or not. This touchpad will show a cursor on the large display and adopt to the touch gestures sames as in VAIO laptops.
I am personally interested in a solution like this, because my highly capable smartphone could become my true thin client for connecting to my virtual machines over the cloud regardless of which client site I am working without carrying a laptop. I would even go further to suggest displaying a touch keyboard as well on tablets when they connect through MHL cable instead of showing the same image as the external display's view. After the implementation of such solution, the right cable could make any display convert into a productivity workstation without carrying my heavy laptop, but a small smartphone.
If all the above is too much to get arround for your current engineering resources, at least turn the mobile device's screen off during MHL mirroring. Converting the turned off display into a touchpad, or displaying the keyboard on a MHL connected tablet can wait a little longer.
I download User Manuals so that I always have them in my smartphone, tablet or computer, ready for me to consult when I need them, without having to go through Sony's convoluted process of finding and opening the manual buuried withing Sony's Support website.
But what would make those manuals much more useful is being able to create my own comments within them. PDF files have allowed users to insert comments since Acrobat version 6 (June of 2003. The current version is 15.) and other companies (Motorola, Nikon, Panasonic, Tamron, etc.) permit users to add comments to their PDF files.