I did some research about this a while ago:
Kate Linder was a stewardess for most of her life (she didn't quit her day job to play Esther), so uniforms are part of her life. She used to maintain a blog - before Facebook Fan pages, and there (in 2008) she said: "Were you surprised about Esther being back in uniform? I was very surprised when I read it in the script. While Esther enjoyed being in regular clothes, she is most comfortable in her uniform."