Top 4 reasons - why I didn't jailbreak my iPhone
1: No Data Privacy - You are at mercy of crackers/hackers. They can steal anything they want from your iPhone, contacts emails, phone numbers, passwords, wirelessly, using your data plan.
2: No Privacy - Rogue apps or cracked apps can monitor your microphone/camera, you know what I mean.
3: Slow Phone - Multiple apps can run in background, all competing for processor. By default Apple doesn't allow this, once you jailbreak, just pray that all apps you run are cooperative and good iPhone citizens.
4: Genuine developers suffer - why would they write apps for free? They will soon give up, typically cracked app downloads are 100 times greater than genuine downloads, overloading limited server resources these individual developers have.