but doesn't jailbreaking remove all the security that ios provides?
Oh, goodness no. Jailbreaking only allows for full access to the file system - by you. Stock apps still are sandboxed and so can't get access to other apps. That's the basic security premise of Apple's and jail breaking doesn't change that. You still can only get apps from the AppStore (you can't legally get them from anywhere else). Those apps are fully vetted by Apple.
Granted, there are jailbreak tweaks that have more access to the file system than stock, but they are built to interact that way, more like a computer. There can be the possibility that someone builds a tweak that is designed to do rogue/bad things to your device, but the jail break community, as a whole, is pretty straight forward. I've not seen a malicious tweak, personally.
Tweaks are just as well vetted - by the repository that hosts them, Cydia (in the persons of the owner, saurik and his staff) and by the jail break community itself. We watch out for our own.
All that said, if you go out to more dodgy sites, or try to pirate (steal) apps or tweaks or load tweaks/apps to help doing illegal and immoral things - well then, all bets are off and you are on your own. There is no policing pirates and thieves and any dealings with them should always come with the risk of damage to your jail break or the device itself.
Bottom line is a jail broken device is as safe as a stock device - so long as you are an honest citizen who exercises due diligence.
That help?
Marilyn