It also lets users feel less resentful or stressed about "having to" upgrade just so they can say they have the latest model. Now, like MacBook, you can just say you have an iPad, and you don't have to explain what sales cycle you're in. Also makes the product base seem less fragmented.
That's not exactly an advantage for Apple (from a marketing perspective)...
Also, the absence of numbers on both the MBP and iPod do make reselling somewhat "harder": "no no, it's the mbp, but the... Uh... Pre-univody version. Yeah, the one with multitouch and a button"
