I regularly use 4 different fingerings for Bb depending on the situation. This issue can be a bit contentious in the sax community. Venting is considered to be best with the standard fingering and some would say that standard and bis are the most optimal choices, while discounting the use of the B/F or B/F# and suggesting the possibility that the latter two fingerings have possible tuning deficiencies. On my saxophones there's not too much tuning discrepancy that would stop me from using any of them. The B/F choice does have a softer sound, but I think that that is sometimes mis-judged as possibly flat when in reality, the timbre is more the difference than the actual tuning. As good saxophonists, it's our responsibility to learn all about the tuning differences from note to note on our horns so we can train ourselves to make the small embouchure adjustments needed to play in tune. I think the fact that all the right hand keys depress the bis key is suggesting that that mechanical design choice was on purpose.