Editing tools on a PC are going to be a lot more powerful than anything built into a DVR, so why do it that way? Also, it isn't really the same concept - 'delete viewed' would be a simple cut (and not so simple as that, but relatively) at the point you stopped viewing. Editing out pieces would require the ability to mark start and end points for the cuts, then splicing the leftover bits back into one stream. Possible, sure, but rather a lot more complex.

Simply cutting we know can happen - you can do it now with MRV. You can start a transfer at a paused point, then stop it on the receiving unit. Then you have a chunk of the program. I've done that to keep just one piece of a show.