Board Thread:General Discussion/@comment-24815992-20140414144050/@comment-11051098-20140414213817

Renji only released Zabimaru once without calling its name after he achieved Bankai, when he fought Byakuya before Rukia's execution. Ever since then, he went back to call its name.

I think it was said, in connection to that, that an attack is always stronger when the Zanpakutô's name is called out, or something. That might be the reason. I suppose Renji did the nameless release that one time just to give Byakuya the hint of having achieved Bankai.