If you are running your code in an iPhone environment and ever want to modularize code. (Say having a tab in someone else's app without revealing your source) Not having XIBs makes using a static library much easier and cleaner.
i am even more anti-xib/nib when it comes to the iphone. they are almost always useless in that case. i've written three fairly substantial iphone apps so far, and i haven't used a nib yet.