Re: the purpose of suburi in aikido?
We do suburi with ken, jo or - this is what we do most often - without any weapon for developing / improving posture, the structure of the body , the connection of the movement to the tanden, timing (of one's own body), When doing suburi with a weapon then connecting the weapon to the body is one element of practice. Things like that.
And - as a result - one aspect indeed can be to move the sword just like so, with no more "strength" than raising the elbows.
To develop muscular arm strength - which I believe to be important also in aikido - we mostly use other exercises: Some of us do suburi with a steel rod or something like that. A suburito will of course help also. But developing strength is not part of the keiko in the dojo. It's something everyone does on a individuel base at home.