I have to disagree with this. In fact, Shelagh is my favorite character on the show.  I think her story has been told in a more complex and thorough way than anyone else's on the show, and I love how she has progressed over the years from the competent, efficient and witty Sister Bernadette of season 1, to the conflicted, lovestruck Sister Bernadette of season 2, to the more unsure Shelagh of season 3 who was finding her place outside the order, to the much more confident, fulfilled Shelagh of season 4. I think Heidi Thomas (the writer) and Laura Main (the actress) have done a wonderful job with her story and I've believed every minute of it.  

As for her voice, remember she did have TB. She did sound a little breathier in season 3, and maybe that's why, but her voice was a lot stronger in season 4.  I think her journey back to nursing was one of more compelling story lines of season 4. Also, her scenes with her husband are wonderful. They had some struggles in season 3, but in season 4 theirs became, in my opinion, the best marriage on TV. 

I guess not everyone is going to love every character, though. She has quite a large fan following, but there are also those who don't like her. As I already said, though, I think she's the best character on the show.  She's been my favorite character since season 2.