machfly I sometimes wonder just how much flight time you have.
I sure as heck had performance information available when I was flying. The POH for light aircraft covers it, fits in the map box and it just isn't that big of a deal to look something up even if you are the only pilot.
Sure, you "do the math" before takeoff but sometimes things change. Weather moves in, the alternate goes to heck and you start working out where to go, how to get there and if you can get there. All that math you did before the flight goes out the window as soon as things change, you better have the performance info where you can get to it.
If this is true the pilot is an idiot.
The POH is in the plane because it's required by the FAA and in case if I need it when I stop some where. I'm aint going to be reading it in flight, there are far more important things to do. If you have to take out the POH in flight you really screwed up at something. You fly the plane, scan for traffic, and look where your heading, you don't fly around with your head down and locked reading a book.
Regarding diversions you do that in your head. You know how much fuel you burn at that altitude and power setting, you know how much fuel you have left, and you can get the rest from the sectionals or low alt charts. There is absolutely no reason to get the POH if you need to divert.