I'm no fan of Spirit's advertising. If I had my way, I would do it different. Nevertheless, it has been proven that edgy advertising like this works. If it makes the news, Spirit's website will flood with hits and guess what? People will buy air travel! I've heard customers criticize but I've also heard many other say it's "funny stuff" and shows the company's "good humor." This is very true especially among the younger crowd, college students, spring breakers, etc.
RyanAir has done the same, and with lots of success. Remember that not everybody gets offended by such advertising. I would speculate that its a small minority. When I saw the Tiger add, I started laughing and texted some friends to check it out. They all loved it and said its hilarious!