Well ... I hate to suggest spending MORE money, but if you think you're going to keep the Series3 units for longer than ~3.5 years, then the $300 for a lifetime service might make sense (versus the $7 a month).
More money now, but after the 3.5 years, its "free".
of course, you're increasing your expenses from 21$ to 900$ now, so its a bit of an iffy equation.
FYI, I've got a second Series 2 I use the same way, no cable connection, use it to watch video from the main unit hooked to the cable. I've been looking into a Series 3 or HD, once I get an answer on what sort of OTA reception I can get in my area. When I get the replacement unit, I'm trying to save up for a Lifetime so I don't have to worry about monthly expenses, otherwise I feel like I'm getting nickel and dimed by everyone wanting only X$ a month (phone, cable, TiVo, internet, etc, etc, etc)