Seems like we always land back on this same subject which is extremely subjective.

Pros for playing in one position. They become very good in one position.
Pros for playing in different positions. They become good in playing different positions.

I think it should be less Black and White and more grey. By grey I think with correct planning and good communication a coach can facilitate a player to play in areas of the pitch to improve the player rather than always playing different positions.