putting any buttons (power or not) on the taskbar is a bad bad idea. I'd rather have the full length of taskbar for my own use.
You know, I actually had a look at the default state of the taskbar/system tray in Windows 8, and realized that, if you put the cogwheel icon there you could even theoretically eliminate the existing system tray icons (or make them hidden), since they are both duplicated in the settings charm.
Start button was not a bad idea but system tray icons and the "show desktop" button introduced in Win7 are surely are. "It's simple, just add a button to taskbar" is a bad design - think years of system tray abuse, weird toolbars by everyone including Microsoft.
If that's considered to be too radical, after all the icons serve to inform you of the network status, signal strength and volume level, so you might want to have them in permanent view (even though Microsoft have decided against that in the Metro environment and have even decided to eliminate the time, so, who knows really) then another option would be to hide the disclosure triangle and only have it appear when you hover over the system tray area. In that case you would end up with the same number of icons in the default state.
Combine the two suggestions, and you could even eliminate all three icons from the task bar. Which you could also achieve by integrating the system tray into the Settings Charm somehow, although Microsoft would probably want to prevent that from happening.
The elimination of these icons would in fact be more consistent with the Metro environment than it currently is, and would, as Dot Matrix might say, "reduce clutter". At that point the task bar in its "natural state" would essentially consist of 3 visible UI elements, search icon, settings icon, the launcher, plus the clock. And since all (including the clock) are relevant both to the Metro environment as well as to the Desktop environment, at least as soon as Metro apps get their spot on the task bar, you could, at that point, allow Mouse/trackpad users in Metro's full screen mode to move their cursor to whatever screen edge they have their task bar positioned at, and have the task bar be temporarily revealed, for increased consistency with the Desktop environment. In fact, if a mouse or trackpad is used as the input device, right-clicking to show the Metro app bar might bring up the task bar at the same time.