I had similar motivations. I don't like it when we have cable-tv outages and I want to continue catching news broadcasts. So I have my system fitted with a Hauppauge HVR-2250 tuner card for the OTA broadcasts, and a SiliconDust HD HomeRun Prime with cablecard is connected to my network switch -- the HTPC system of course is a workstation on the LAN.
I have something similar setup as Bonzai. I use a SiliconDust Prime 3 tuner cablecard tuner that shares 3 tuners to all the PCs on my network. I have 3 HTPCs in my house and I supplement the cablecard tuner with a few ATSC tuners. Heres why:
1 - one HTPC is on wireless, another is on powerline networking. They work most of the time but can hiccup now an then, and while cat5 is on the to-do list it hasn't happened yet. With a local ATSC tuner connected to a roof antenna, local shows can record OTA as a priority over the cablecard tuner and not be troubled by network bandwidth.
2 - WMC can balance the number of recordings vs the number of tuners on a single PC (it knows not to schedule 4 things to record when there are only 3 tuners), but it doesn't know what the other PCs are doing. So, if I set 3 things to record on one HTPC and 1 to record on another, WMC will go to start the recording and one of them will be a tuner short. You won't know until its too late. Prioritizing OTA tuners can sometimes save you if one of the 4 shows was an OTA broadcast show.
3 - While I've not had to deal with this personally, network blackouts arent really an issue when you can just tune with an OTA tuner.
4 - I can pick up an ATSC pcie or pci tuner for around $10/each on ebay. 3 extra ATSC tuners is around $40 where it would be an extra $120 for another 3 card tuner, plus the additional rental from comcast for the cable card (and will inevitably eff up my bill in the process).
5 - Whoops, forgot one more. Sometimes the cable company will set shows to be 'copy once', meaning you can't play them on anything but the original device they were recorded on. OTA shows never have this. So even if its something on FOX or NBC, recording it over cablecard will sometimes net you some nasty DRM, recording the same show OTA leaves you free to copy it around.