There's no reason the DECA wouldn't work for this however if you start to have issues with your MRV performance this would be the first thing to remove. Adding traffic that was not intended to be there is obviously not going to be something supported and can cause issues. YMMV and I'm sure people will use this as a way to get around running an ethernet cable but for a long term solution I would say the cost of running one ethernet cable would be better.
I see no real reason to spend the money to run ethernet. As far as I see it, my Blu-ray Player is going to be requiring no more bandwidth than the HR21 would (probably less, in fact), and it's not like it is possible for me to watch shows on both at the same time. Maybe there is some QoS that gets screwed up by putting the switch in the path, but until I see issues occur, I plan to proceed as is. Running cables at my house is an absolute nightmare (no unfinished basement, all ceilings drywalled).