Jailbreaking does not magically leave your phone wide open for attack.
Yes it does.
Jailbreaking requires you to write code to modify the bootstrap sequence. If you cannot do this, you must TRUST someone else will ENTER YOUR SYSTEM, and then LEAVE. You TRUST the program(s) they left behind (Cydia or equivalent) do not have any holes (like enabling telnet/ssh/trojan/something else bad by default) or, if they do (Strike 1), they will update their code appropriately and quickly (Strike 2 if they don't).
And that's only if you now use your phone as if it isn't jailbroken. If you want to install unsigned apps (reason most (everyone?) jailbreak), you leave yourself even more open to attack every unsigned app you install. That isn't to say you safe via Apple's signed process, but you are definitely not *safer* than without Apple. Unless you took *additional* precautions after jailbreaking, you must worry about leaving your phone anywhere as any technically minded person can get root access to your phone and install any sort of "bad" software!
Saying jailbreaking does not magically leave your phone wide open for attack is incredibly naive!