I don't know why I'm feeling slightly offended for Shiemi with how many people dislike her...but whatever. I guess it depends on persepective and personality. So it's fine. Which is why I suddenly feel like saying this (probably ridiculous, unnecesarily long) statement due to the fact that I really like her:
Personally, I think Shiemi's existence is pretty important and no she's not useless, stupid, etc. etc. She's a catalyst for several important events, she's capable and able to grow, and she's got a pretty big heart - which while it does make room for rash/irritating things to happen - it also allows her to change and understand and be able to keep up with the Okumura bros and everyone else and not be afraid of them. Which is a pretty big deal. Especially when it comes to Rin. She's someone he trusts and will listen to, and whether you like it or not, her thinking actually kind of matches with his in a way. She really pays attention to him and will notice some things not everyone will see about him, too. Plus, she cares for him unconditionally.
Which along with every other relationship/friendship Rin has, that's really significant and important for him to be able to have. And it doesn't hurt to give him someone cute to crush on either, so... ;P