My needs have been, so far, either fully portable or no need for required portability. So I have my home rig which isn't portable and for my fully portable needs I've only found need for my iPod Classic line out paired with a small portable amp. My RSA P51 Mustang works well for this as I'm sure the AirHead would, especially since I use IEM's for full portable sound (I went RSA Mustang because of the impressive battery life). When I'm in a plane, in an airport, on an outing or in the office where I move around, I can move around easily, and I really don't need that extra special sound. My portable setup is fabulous under the circumstances, background noise, activity etc.
The Portable Micro and Portable Desktop Amp/Dac(s) are to me, semi-portable options. In this situation, you wish to move your rig around but not too much, so having too many components plugged in is very convenient. You also are likely to have an environment where you can place these more bulky components and connections, as well as where you are able to really listen to and appreciate that extra sound quality with very decent cans. A patio, hotel room or quiet office where you spend a lot of time at your desk undisturbed and where power outlets are either absent or at a premium make for typical situations.
If you choose a semiportable solution for full portable duties, you'll feel awkward and end up not listening to your music as much as you would if you had the proper solution. Finally, don't under-estimate the currently available fully portable solutions. They sound *great*.