Until he comes up for air, check out his videos of offroading his Ural outfit. Your "need" to stand is only a function of the difficulty of the terrain. While the second driven wheel is certainly an added benefit, it is by no means a necessity (unless you already own one and can't imagine not having it!). The caution comes from your desire to have more power after the KTM 950 experience you've had. Any hacked bike will give a lot away having to drag that chair around. On a Strom or a GS or something like that, it won't be such a deal. Smaller bikes and older Urals...well, there are a variety of chairs, from the heavy Ural/Dnepr to the ultralight home-built enduro rigs...you have a lot of room in there for power-weight ratios. Gearing for power is another consideration. Chain-drive bikes make it simple. Shafties limit you to different tire/wheel diameters or expensive gear changeouts.
Of course, the nurse in me wonders why you just don't go bionic with the knees!