So based on these scenes we can conclude that the captains assumes the following regarding their method of sealing.
[list=1][*]Only one Bankai can be sealed at a time.
[*]The seal can be broken, and that learning how to break the seal would lessen the advantage the Quincy have over the Shinigami.
[*]The sealing requires a period of time which they can use to weaken or potentially kill the opponent.[/list=1]