Some people, like my daughter hate spiders. I don't mind them--after all, I could crush them with one step. My phobia? Snakes. I prefer not to see the slithery vipers on my hikes through the hills, and am ever so happy that they are reclusive animals.
Are there any critters that make your skin crawl? I think adding a phobia is fun when building a character--Have them fear something, and then have that thing plague them or watch them overcome their fear. I know this isn't anything new, but it does add a dynamic to your characters--and really, how many people do you know who fear something? A lot, I'd bet. I have a character in my current WIP who can't swim. Her inability to swim causes problems and the fear paralyses her at a critical moment. Will she fight or will she fail?
Write on friends!