Theoretically, sure.

In practice, it tends to be used the way I used it. And honestly, yes, there is a need for distinction between the two. The fact that this very thread exists is proof of that.

Bisexuality is the attraction to both/two sexes, rather than all sexes (pansexuality) and it's just plain misleading to argue that there is zero biological difference between a transgender woman, and a cisgender woman. Hence, a need for distinction. We can all be as open minded as we like, but I can't change my sexual preference to include trans people. Therefore, I use a label to express that, and it's commonly understood to mean exactly what I mean when I use it.