Although this event is generally considered a bad thing, it could be considered a good thing in the following ways:

(1) It allowed Mulan to discover that not all of the enemy forces were dead, which quickly re-energized her after feeling down following her abandonment. If she had been with Shang and Co. when the remaining enemy forces attacked at the palace, those enemy forces could have seriously injured and/or killed her while they kidnapped the Emperor.
(2) It allowed her to defeat the remaining enemy forces under her real identity instead of an assumed identity, which not only redeemed her reputation but earned her lasting respect and trust (in contrast to the respect and trust that was shattered when her true gender was uncovered).
(3) It earned her a suitor from the military (and this suitor also happened to be the same person she had fallen in love with while disguised as a man), which helped fulfill her family's wishes for her to have suitors, after she had been written off by the matchmaker earlier.

^I 100% agree with you.
On the long term it was of course a good thing as she can live her life without having to pretend to be Ping/Ping's sister etc, and she could take credit for her actions herself.
But naturally it would have been bad thing if Shang didn't spare her life.

I agree with the first comment, but might also add that - even though the movie's events shows no evidence of it - in theory it could have had some impact on the way in which women are treated in her country. Mulan is brought up in a society where females are judged on their abilities to be a good wife and mother, and while there's certainly nothing wrong with a woman being good at these things, everything is wrong with treating them like that's all they're good for. Mulan defied expectations and proved that gender should not be a factor when recruiting for the army.

As I mentioned, the movie's events do not show what kind of an impact this had on society, if any, but the possibility is there.