Why not use a small, hi tech, water cool diesel auxiliary power unit, as needed, for heating/cooling, limp home mode, etc. ???
Because it uses diesel.
People who want diesel cars, buy diesel cars.
Use the turbine used in the Linkvolt.
This is like saying why not add an electrical drive system to every gasoline car for "limp home" mode etc. Yeah, I know that my statement sounds stupid at first since anyone can counter that it's unnecessary given that you can supposedly refill your gas quickly and conveniently almost anywhere.
Well let me tell you, after eight months driving the Model S and occasionally having to drive an ICE car, I find ICEs quite inconvenient.
Yeah, I get that an "APU" as you call it might give you some extra margin for error, etc, but it would add a lot of cost and complexity, and I would probably never ever use it.
Instead, it's like my phone. I know the battery can die, so I am just remain conscious about it. Especially if I am going on a trip or something, I pay attention to the battery life and I try to keep it charged up when I can and avoid playing games on it, if I know, for example, that I'm going to be walking around a park all day with no access to recharge.
Many of the people who have purchased plug-in hybrids attempt to drive in electric mode as much as possible. They don't want to use their "APUs." So why drag around the weight, deal with the gasoline/oil and complexity? Electric all the way!