Roku excels in having lightweight hardware that serves its purpose without anything extra (hence the low cost). Having a mic and webcam (or even just support for them) would bulk up the box and the cost, and honestly, it's not something the Roku target audience uses.
Use a computer.
I agree. But even a computer is no longer needed as this http://shop.skype.com/phones/phone-adapters/connect-me/
will allow one to use their regular house phone. There are also phones that are stand alone Skype devices.
One thing I cannot understand that many people see to want is the video feature. There is no one I talk to on the phone that I want to also see at that time and even fewer that I want to see me.
Roku is completely the wrong device for this kind of use.
I might add that anytime a device tries to incorporate a lot of different functions it generally becomes practically useless for all of them.