Re: benefits/ disadvantages of tensioner-less chain?
The best thing I ever did to my bike was remove the tensioner. It seemed I was adjusting that thing every ride, it was very frustrating. Not to mention all the noise and drag it added to my bike, specially when pedaling without the motor running, the drag it added was simply unacceptable to me.
I adjusted the motor mounting angle a bit and put a very small shim between the motor and rear motor mount to tighten the chain just a bit and never looked back. The ride is super smooth and quiet, there is MUCH LESS drag when pedaling, and I have never once had to worry about my chain tension since I got rid of the tesnioner....
Some people will chant you down, tell you that you need the tensioner for adjustment later etc. I put over 500 hard ridden miles on my bike and the chain tension was still fine when I disassembled the bike for the winter tear down. If you use the kit chain, or any bicycle chain for that matter perhaps chain stretch might become an issue requiring a tensioner, but if you run an ANSI pre-stressed industrial #41 chain I dont think it will be an issue. These chains are MUCH stronger than any bike chain, and they are pre-stressed/stretched before you buy them so stretch shouldn't be an issue IMHO.
Not every bike can be setup to run without the tensioner, but if you can I would encourage you to get rid of it. If you can't I would suggest you look into the rollor blade wheel tensioner, they seem to work better than the stock plastic thing...