Sure.
but to convert a digital signal to analog, only to re-encode back to digital to store it ...
It comes off the wire as digital, but most TVs are analog (that's why a cable box is needed for digital cable). So you must have a D/A conversion in there somewhere. The storage is digital because that's the best way to preserve fidelity while minimizing storage space.
I do get your point - it would be good to be able to feed digital cable into a PC, do the processing/storage magic, then send digital to the TV. Those days are coming, but will likely be encumbered with DRM baggage.
To get digital video from cable into the PC, you'd need something similar to the cable box. If it could have been put on a PCI card for a reasonable amount of money, it might have been done already. But with the DRM stuff coming, I doubt that unencumbered stuff will be easily available in time.
Since nonHD TV is rather low resolution anyway, the D->A->D->A conversion train shouldn't really cause much of a problem (if the PC can keep up). Once HD takes over, then digital inputs to the PC will probably be required. So you'd go: HD receiver/cable-box -> digital PC input -> digital PC output -> HDTV.
My guess, anyway.
Cheers,
Scott.