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Always having (and desiring) the newest thing is the blessing and curse of being an early adopter. You get to enjoy period of having the latest and greatest technology, but then a little time passes and what was late and great no longer is. As long as you can afford it, and have prioritized ownership of the latest iPhone/iPad/whatever as something important to you, I don't think that there has to be ANY justification. To paraphrase Descartes, "I want, therefore I get."
Currently, most of the information about the iPhone 5 is based on speculation and rumor. One rumor that has been repeated though is that the IPhone 5 will include near-field communications (or NFC). This could make the iPhone effectively your wallet, potentially allowing mobile payments to take place with no need to take anything out of your wallet. Neat, if safe and adopted by most merchants. We will see what else happens the iPhone 5 will offer. I'm sure it will be neat, but from a purely economic perspective, replacing a 4S in less than a year is not the path to increasing your 401K. The real question is do you care about that?