Using a phone has some disadvantages:
Non-clean sound due to implementation of electronic design
Extra heat from other components
Interruption listening to music due to calls and notifications from data elements.
General storage limitations (some better than others)
Huge hit on battery life.
Updates to a phone may or may not interfere with SQ elements. SQ can be affected by updates to DAPs but they are all focused on one thing. Sound.
Unique factor - not mainstream
Price - performance - Longevity - DAPs generally (at least in my experience) last longer than phones. You get rid of phones and have different Daps for different reasons depending on uses.
A giant touchscreen can be annoying when you only want to listen to music. (personal preference)
Generally easier to listen to as there are generally tactile buttons to use while DAP is in your pocket and such.
Well, for most of your examples, you seem to be directly comparing a phone (seemingly not the best one either) with the DAP. The SQ won't be affected by the phone (or at least very minimally) in the case that I suggested as it would be digitally sent, then converted and amplified, by an external DAC/amp. The benefit you do have however on a phone is practically any UI of your choice.
Also, when streaming the music to an external device, it really doesn't take much battery life from the phone. The limitation is once again on the DAC/amp you're pairing it with. I don't see how you missed that part I mentioned. Of course the phone on it's own doesn't even compare to a dedicated DAP when it comes to music listening and quality. It's only recently that, imo, the gap has been closed with this potential implementation of DACs through USB.
Finally, I just have to mention that music listening being interrupted by phone calls would happen regardless, just in the case you're describing, it's with the convenience of being one click away for answering and you don't have to remove your headphones. Before smartphones (or even when they were first introduced and just weren't good enough), I dreamed of the possibility of not having to very frustratingly pull the earbuds out of my ears and hold them in one hand (or throw them over the shoulder) while I hold the phone in the other.
Sorry but, none of those points I didn't consider and their implications are really minimal even in the cases where one of those benefits may apply considering what would fairly be weighed against that. Not to be entirely dismissive of what you said, but the awesomeness of the X5 (I guess in a good way) has simply deluded some people. For the average person, it simply isn't practical anymore, even for those interested in ultimate SQ, considering the alternative options available for that now. Even for me who has all the reasons in the world to want to get something like this - I'm having a hard time finding reasons to invest in it. It would just sit there being pretty for the reasons I've expressed, mostly that there's not much if any at all difference in SQ and it's less comfortable/convenient. The only plus is that it doesn't have to be connected to your phone directly through USB but that doesn't bother me and the freedom from that isn't worth what you'd take away (better tactile buttons and a better UI of your choice) - also not having to switch your headphones out for your phone. Also, there's an obvious price difference between this and alternatively great portable USB DAC/amps.
Edited by Typhoon859 - 3/4/14 at 3:48pm