One bad thing is that if something happens to your iPhone and you have jb it then you can have your insurance fix or replace it though am I correct? I have insurance on mine it's my security but I may be wrong and maybe you can still have insurance on the jb ones.
well if by chance you do have something go wrong then you restore (if you can) then take it back to apple..
They do not touch jailbroken devices but a simple restore and thats all traces of the jb gone, I am not sure about 3rd party companies such as protect your bubble if they will still do so.
But firstly out of the 1.5 years I owned my iphone 4 and the 4 years I have owned iphone devices not once has anything gone wrong even after always being the fist to install jailbreak while others have endless "problems" they just are not 100% sure and they see something they are un sure of and panic and straight away think they broke their device or "bricked" it and they start over again by doing a restore, you never really break your device by doing a jailbreak, it is safe and Jailbreaking was given a bad name from Zibri who was a terrible coder and lots of people had problems due to his app.
Not only that but people take these free jailbreak tools and sell it on and claim it works with firmware and devices that it does not, and on top of it not every one is sure of the difference between jailbreak and unlock so they rush into it not even knowing what they are doing.
but jailbreak is not for every one and not every one cares about customisation of their device and it works just fine with out it. In fact I dont care if my device is jailbroken or not but its still fun to mess around with and brings back the fun of the device and its something new that you can do to the device..
A few apps that really help people like being able to facetime over 3G allows families to talk to each other eg. fathers or mothers who are away a lot and use facetime as a way of keeping in touch who dont always have access to a wifi spot...
so many similar reasons why people would do it and how it has helped. Jailbreaking is not the evil that some people make it out to be and all its really doing its turning the disk into "writable"