Well, i understand your point and its certainly the reason why Microsoft is heading this way.
But there are many issues why i don't agree with this decision. First of all, you assume that the tablet is always used in battery mode. But i guess if people are doing "real work" they are doing it mostly on their desk or somewhere in facility where they can plug the powerline into their tablet.
I also think that people will be smart enough to recognize that some tasks needs more power than other tasks. But at least you should have the choice to decide what you want to do.
So, if you are running Windows 8 in metro mode in WOA, why not disable alle legacy apps/services and only allow them in desktop mode. This way you could ensure that battery usage in metro mode is fine, but also allow the user to use their known and loved Windows apps if they need them.
WOA has to much restriction in its current form and there is no added value in comparision to android tablets. Instead it will heavily lack on number of apps. Sure, in the enterprise area WOA won't play a role, because Windows 8 on Intel tablets will be the choice. But Windows 8 experience is all about end consumer! Lots of people won't have any reason to use WOA over android tablets, because either they are using WOA or android they won't be compatible to existing Windows software. But android provides hundred thousands of apps. I really really doubt that the number of Metro apps will outperform android apps (for a long time).