It's about exposure. Maybe you think it's insignificant, but it isn't. Would it be better to have a well-written main character who is trans? Sure! Guess what - that's not going to happen, at least not yet. What can happen, right now, is for Capcom to state unequivocally that Poison is transgender, and that would be a step. Until we have small steps, we can't have big ones, because nobody is willing to stick their neck out that far.
It's like how you had maybe had a side character in a TV show be gay every now and then, but not very prominently and not very often. And then, in the middle of her show's run, Ellen comes out as gay on the show. It was already an established show, so it wasn't trying to launch itself as being a show with a gay main character. But that event no doubt set the groundwork enough so that Will & Grace could be tried, a show were they did launch right off the bat telling you that two of the four main characters were gay. And now, while some people still do get bent out of shape about it, that concept - of having a gay or lesbian main character - isn't the taboo that it was before. But had those steps not happened in progression, later steps might not have - at least, not back at that point in time.
Yes, it's hard for people outside of the whole transgender issue to understand why this is so important to those who are. But that lack of ability to understand doesn't mean that the side who want Poison to be trans or wrong, or that they're silly for wanting what they want.