"why have the tank move?"
I think the real question, is why do things move in the first place?
"What is the real reason for the movement?
Is it to withstand windshear in heavy winds?
Is it to reduce vibration down through the bowl and thus noise?
Is it so that the rubber washer is compressed to prevent leaks?
If you're paddling down a river in a canoe and a wheel falls off,
How many pancakes does it take to shingle a dog house roof?
Answer,
Three, because ice cream doesn't bounce.
Oh, and now there has to be a reason?
Does the jiggle in Jello have a reason?
Do corndogs really need mustard?
Why do some chairs rock?
How come some cars use torsion bars and other cars use McPherson Struts?
Is it okay to use a "swing axle"?
Could all things be made the same?
No, I think there may be ownership of ideas
http://www.cptech.org/ip/wipo/
But a lot of ideas can be copied, just look at how much Toto is copied.
They could make a tank that is
"rock steady" and have a bowl that flushes really well.
Some brands have tanks that feel more solid and don't flush well.
Some brands have tanks that really move and still don't flush well.
You don't need to worry about it,
but you will.
But then, maybe there is a design opening somewhere for someone that can create a quick bolt platform that hides between the tank and bowl using something like kevlar fibre wingnuts. Something invisible when the two parts are completed, sort of a marriage between two seperate entities, creating a super oneness of being and intent.
Maybe a trip lever that slides down as you push, rather than a lever. That way you could just palm the lever and slide it down sort of like a dimmer light toggle.