Rather than carry on with the inane discussion above I decided to test my particular MHL adapter, the one sold by T-Mobile... I'm using Battery Monitor Widget and a beefy Monoprice HDMI cable btw. Here's what I've found:
A) It clearly states "Charging (USB)" under About phone/Battery, which means the phone WILL NOT charge at full speed when plugged in via MHL... At least with this adapter. USB charging usually means the phone is getting a max of 0.5A from the port (which is what you'd get from a PC's port), which is half what you'll get from the stock HTC charger or any other 1A charger.
B) With the phone completely idle, while connected to Wifi and with the screen turned on but at low brightness (30%-ish) I usually see a power draw of around -250mA. (ideal conditions really) Any time I plug my phone into the stock charger with any microUSB cable I've tested it jumps up to anywhere between +750mA and +800mA, e.g. max charging rate of 1A minus the existing draw of 250mA-ish. When plugged into the MHL adapter with the same charger/cable I see around +150mA, so technically it's charging but it's not even charging at 0.5A (more like 0.4A), it's charging very slowly basically.
C) Playing Green Hornet under the same ideal conditions (Wifi on, brightness low, no streaming) I'm seeing a draw of like -10mA or just under that, so it's DISCHARGING albeit very very slowly. Basically it's discharging at a negligible rate, slower than it'd discharge in your pocket while waking around and pinging different 3G towers.
D) Finally I tried a little Youtube streaming with a 10 min. clip (linked below) which seemingly resulted in an ever increasing power draw which peaked at -380mA (literally it kept using more and more power as the clip played, the Battery Widget graph was a sharp downward sloped line). It's still not discharging THAT fast (about as fast as normal Wifi web browsing would discharge it) but it's discharging at a pretty good clip nonetheless.
The takeaway from all this is that MHL is hardly ideal, at least with the adapter I've got. Any heavy streaming (Netflix etc) will discharge your phone at a decent pace and if you intend to watch a full length movie I'd resort to some of the measures mentioned before (airplane mode, low brightness, etc.). The MHL/HDMI adapter wasn't really looking all that great to begin with because having the adapter hanging from your phone with two cables hanging from the adapter itself is pretty awkward imo, I'd rather just use my tablet for this kinda thing since it has a much beefier battery.
Now if anyone has any MHL adapters that report "Charging (AC)" when connected, that'd be a different story, since it'd mean at least double the charge rate which should be enough to keep up with basic Wifi streaming (4G might be a different story).
Now I'm curious whether TVs with actual MHL ports supply 0.5A thru MHL cables or whether they do 1A, MHL looked like a great long term single cable solution that eliminates the need for all these adapters (with future TVs), but it's gonna fail if it maxes out at 0.5A. Hopefully it's just a problem with these adapters (which themselves seem to require about 0.1A)
---------- Post added at 04:47 AM ---------- Previous post was at 04:35 AM ----------
Some quick Google-fu revealed some bad news... The MHL spec itself seems to be a signaling spec and isn't necessarily attached to any single connector type, sorta how Thunderbolt is currently being implemented via Displayport copper cables but may move to fiber optic cables in the future. So there's room for change. The bad news is this tidbit:
"Operation of the mobile device in MHL mode while 5 volts and 500 mA of power are simultaneously provided from an HDTV or other CE device"
So yeah... Dunno what genius thought this up, but as currently implemented it's gonna fall short when used on modern smartphones. That quote's from Silicon Image, maybe LiquidSolstice can clue us in on the reasoning behind the 0.5A limit since he's done work for them... It seems woefully inadequate from a user perspective tho.
SI was only one of half a dozen companies in the MHL consortium that developed the spec btw (Samsung, Sony, etc are in it too). It's possible they stayed within standard USB 2.0 limits for power supply to retain compatibility with cameras and other devices, hard to say, the same blurb quoted above can be found at the MHL Consortium site. Good old microHDMI is looking a lot better right now...