False. It removes the sandbox restrictions on apps. Anything you download can now possibly have access to the system settings / files that are not available to unJB'ed phones. That's how jailbreaking gives you access to a lot of the "super cool features". Unfortunately, it's also a huge security risk.
Plus, you're trusting the developers to not inject malicious code into the apps you're installing.
If you can't see the potential for attack vectors in either/both of those things, there's not much I can do to explain them more.
It poses a security risk for people that don't know what their doing. I don't install jb apps from sources I don't trust, make sure apps are always compatible with my device and FW, and dont pirate apps.
I'm involved a lot in the jaibreak community and know what to install and what not to install. Plus, there are moderators of the repositories that host the packages that remove all harmful apps, tweaks and themes should they pose any risk.
Just like on a computer, know what you're downloading and you'll be fine.