Hi and Welcome, Jarhead,
Why indeed....in my opinion, the XS650 is all about the engine. It is iconic in the same way a Triumph Twin engine is---it is a classic example of "what looks right works right". It makes it easy to use it as a basis for a custom, a cafe, leaving it stock, or even, like one of mine, a sidecar tug. But whatever an owner does, it remains all about the engine.
When it's all said and done, I believe the XS/TX650 engine will rank up there with the Honda Four, the Kawasaki Z-1, the Bonneville and the Norton Commando as a landmark of beautiful engineering.
Just my opinion, but I bought my '77 in March of 1978, and still have it. Of the dozens of bikes (mostly British) that have gone through my shed, the Yammie is the one I will never sell. Go figure.....