After 3 years and 3 iPhones, I jailbroke my 4 for one reason: I'm sick and tired of the same 6 SMS alerts and ONE email alert. It's complete and utter ******** and I refuse to be a slave to Apple's Pavlovian marketing ploy. I mean 3 years, same tones?! Why?! What are they thinking?! I just can't think of a way to justify it without painting Apple as an evil company.
After jailbreaking, I realized that there's also quite a few other compelling reasons, AND my phone is just as wonderful and stable as before. I highly recommend it to anyone.