I think having Dottie be unsure about their marriage from the beginning and then kissing Jimmy would not have let the ending that we ended up getting be very satisfying, because it would have seemed like she was ultimately settling for this guy she was never sure of and didn't pursue other avenues in her life that she secretly wanted.
I actually prefer it as it is, because I read it as Dottie being a more typical 1940's woman who, even though she actually is passionate about baseball and playing, simply didn't see herself as someone who was going to have a life that was meant to be any different from most women of that era, and not reaching for it because it just wasn't expected of her. She loves Bob, she wants a family and she doesn't consider doing something different with her life outside the war. And I can believe there were many women of that era, especially around her age who were that way, despite hidden or buried talents and passions. A lot of them were asked to step up in different ways during the war and then asked to go right back to being homemakers after the war was over- I'm sure there were women who were resentful of it and others who didn't question it because this was as society was structured at that time. Perhaps in retrospect years later they might look back on it and realize, as Dottie does, what could have been different.
Having all that other stuff in the movie makes it very explicit that she's suffering and repressing who she is and regretting every choice she's making in the moment and that would have made the ending much more depressing overall. As it is, it's more bittersweet, because it's all in her memories of it.