I did read the built in amp on the Iphone 5 isn't quite as good as the Iphone 4/4S. The cost of the dock adapter isn't prohibitive, but the amp option is a consideration as it does offer somewhat of an improvement, particularly with some additional bass impact. I agree it is not necessary, but a nice option.
The cost isn't prohibitive, but it'll be huge to carry in your pocket! Also, we don't know how nice the DAC in that thing is yet. Still though, even if the amp in the iphone 5 isn't quite as good as the 4/4S, it'll still probably be good enough for portable use. For home use, a desktop amp would probably help, but you probably won't be able to tell the difference outside with all of the noise going on around you!