Re: Two items that could save your life
I've accrued thousands of miles on a 'properly installed' rag joint. Never bent or loosened a spoke and I can assure you my chains and sprockets are perfectly aligned.
I have one bike with a sprocket adapter and I'm not all that impressed. It might help when you don't want to take the time or have the patience to install the rag joint correctly but above that, the adapter was an over priced accessory that I could easily live without.
As far as a chain tensioner, I've used them and run without a tensioner. Again, 'properly installed' a tensioner can provide you with a lot of riding without a problem. A spring tensioner is only a device that provides a way to tension the chain without doing the labor of adjusting the rear wheel. With an adjustable tensioner it only takes minutes and and there's no chain slack on start up or deceleration as with a spring tensioner.
Sorry I have to disagree with your assumption that they are critical to a successful motorized bike. As I posted above I feel that good brakes are far more essential than the two items in question.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"