u should unjailbreak for the following reasons
-get ur warranty back, cuz if something goes wrong where its completely busted while its been jailbroken, ur out a few hundred bucks.
-get ur battery life- depends what you have installed i took of winterboard and see a big improvement stil have sbs settings on my phone
-avoid slow phone - my phone seems fine from day one.
-avoid the iphone worm again to avoid the SSH worm change your SSH password
-stick with apple's defaults to ensure ur safe use of the iphone what do you mean by that?
-avoid downloading illegal/pirated apps again you have to put in a right source and install certaint programs
-avoid getting hacked, which is possible only on jailbroken phones