NRS players are funny because they always try to act like the most popular characters are overrated and meh. There is next to nobody as popular as Scorpion, Sub-Zero, Mileena, Kitana, Raiden, Liu Kang and Jax. They're mainstays for a reason, people love them and NRS clearly feels they need to be in nearly every title one way or the other. Even when they miss the "original" game nine times out of ten they get added in all expansions and this has been the case for all of the above since MK2 onward (UMK3, MKGold, Unchained, UMK11, etc) and this is with few exceptions like MKVDC.
If people were talking about Jade, Baraka, Smoke or other characters that NRS are hot and cold on that might be valid, but not for Mileena or any of the MK1 or MK2 mainstays mentioned above.
NRS and the community have their must haves and Mileena's popularity seems to have grown to her being considered a "must have" akin to the other aforementioned characters.