We have an XBOX 360 in the living room for out main set top box.
Amazon Prime + Hulu Plus are plenty of TV for us. Axed Directv over a year ago.
We also have a Roku3 for other rooms in the house, (bedrooms/office)
I just bought an extra power cord and HDMI cable for my "office" (hobby shop/gun room)
But yeah, you don't need a Roku hooked to the same TV as another device.
Hell, you can actually access the same stuff on a PC as you can a Roku, but the Roku gives you a MUCH more streamlined menu system/playback capability/etc.
IMO it's worth the $80 one time purchase vs tying up my laptop to watch TV