I'm unclear how jailbreaking impacts your device security in a meaningful fashion. All it does is give you the possibility to install something that could cause you trouble, but absolutely nothing on its own.

Most of the jailbreak apps are open source anyways. (MobileSubstrate and the majority of things that use them, for instance)

Pointing at any kind of positive to the locked-down-ness that Apple strives for, regardless of its truth, personally makes me feel very wrong as someone who values openness.

Edit: I love how asking a question is grounds for being downvoted into the negatives now.