When HDMI spec was defined, working group must have thought all the video sources (like DVD players, STBs ...) and display devices (CRT/LCD/LED Monitors/TVs) will have their own power source. So they did not consider supplying power in either direction.
Now we are having Android Dongles, ChromeCast kind of light weight (in terms of power ) video sources, which can be easily powered by display devices.
This is understood by the time HDMI 1.4 is defined.
There must be good reason why, but supplying power was not considered even in HDMI 1.4 / 2.0
For this reason, chromecast and android Dongles (and MHL cables) takes their power from USB ports. This USB power may or may not be from TVs.
Supplying power from display devices is inevitable going forward. So we should quickly find an alternative.
- - - - Proposal - - - -
The mechanism that is used in RJ45 to extend 8P8C to 10P10C can be implemented.
The existing pin 18 shall be used for this purpose by increasing limit from 50mA to say 2A)
a new connector with HDMI and microUSB next next (similar to USB3.0 using similar connector as microUSB). so that existing HDMI cabels, micro USB cables can still be used.
All the new Display Devices / Video Sources can implement this. and they will be backward compatible too.