Greetings from an (expletives deleted) who has jailbroken all his iPhones to date. Yes, it makes them more open to hackers - but since I don't use dodgy software, it's a calculated risk.
There is software that I want to run which Apple has decided I should not be allowed to use. Curiously, it was allowed for a while, then Apple elected to disable it (without warning). The choice is therefore iPhone (which I like) and jailbreak, or one of the other phone varieties that will run that software without needing anything resembling a jailbreak. I tried Android and Windowsphone and didn't like them.
As ever, broad generalisations don't work in every case. I can't see how a hacker is going to get into my iPhone - or indeed, how not being jailbroken would make any difference in my case anyway.