The lack of mix-and-match is a bit unfortunate. My parents are using a Premiere in dual mode, because their condo provides them free, standard definition, analog cable and they supplement this with an OTA antenna to get high-def locals. I don't know how common this usage pattern might be, but TiVo has basically killed it.

(I suppose you could buy *two* Roamios, one for OTA and one for cable. But somehow that's just wrong.)