I've always said that I love Shirley as a character, but I'd hate to have her as a friend or a relative. I think Shirley is actually written quite consistently, but she's so flaky that it seems like she's all over the place.
Take her unending support of Dean when Linda's rape became public. Shirley had fought to have Mick back in her life and was aching to be a mother to him when boom! She turns on him and Linda and throws all of her support at Dean. While that seems very inconsistent, it's actually very characteristic of Shirley. She has long thought that of her children, Dean was most affected by her abandonment (which is probably true). Her guilt over what Dean did to Linda made her feel awful for her shortcomings as a mother, so she overcompensated by immediately taking his side and leaving her relationship with the more stable, less needy Mick to sour.
Linda Henry also hits it out of the park with Shirley. She can stick around as long as she likes in my opinion.