|I've found that amps are in most cases unnecessary in portable rigs. Unless you're hell bent on using hard to drive cans or squeezing the last 1% of performance out of something. I only carried an amp with me and used it for all my listening early on when I had my iPod Video > T2 > UM1 rig. After I got the better earphones, the amps only got used when I was sitting down somewhere for a while *and* paying attention to music (rather than listening to it in the background).
i totally disagree with everything you say until you get to your last line, the background verses paying attention.
When i'm working I listen to ipod and phones without the amp. But again, i'm not really paying attention to the music so the little things like vastly improved dynamic range and substantially better crosstalk isn't something i'm enjoying. Its portability at that point, and keeping the volume low enough to hear things around me, which is more important to me at that time than quality of sound.
BUT, there is a night and day difference in the sound when I add the amplifier to my rig and my livewires are pretty far from the 'hard to drive' side of life.
My rig is 5.5gen ipod, LOD, XIN, livewires: hardly high end audio components and when you say that my amp is only pushing that last 1% of performance from my ipod you are misleading, unsubstantiated and simply wrong.