BestConfigs: Home Theater PC

I'm thinking about building a new HTPC. None of the recommended systems seem to include a TV tuner card, such as a Hauppauge. I've been using an old Dell PC as my HTPC for a few years, along with the TV card. It works quite well as a DVR, and I would like to continue to use my HTPC as a DVR (and also for live TV, although that's not as important). Is there any reason why the builders don't include a tuner card? Is it possible to have DVR functionality with one?