I have a HR21 and a R22, but I don't see any use for this feature. My R22 is hooked up to a 32" SD set. Why would I want to watch anything from my HR21 on it? And why would I want to watch recorded SD material from the R22 on my HDTV? If I want to watch recorded HD programs, I'll record them on my HR21.
I guess if I had a TV in my bedroom connected to a non-DVR receiver, then it might be nice to watch a program recorded on one of my DVRs.
As is, even though I have the HR21 and R22 both connected to my home network, I see no reason to pay D* another $3/month for this service.
Your R22 can record HD programming since you have another HD receiver on your account. So....if you wanted you could share HD recordings. I have four HD receivers, but only two HD Tv's at the moment. I share programming back and forth between all four receivers. Worth every penny of the 3.00 IMHO....