The main reason I prefer the purpose-built holder is that, in this case you don't have to connect the power wire after you connect the phone. I'm hoping to also attach my headphone cable (connected to the car stereo) directly to the mount so that both will be connected when I drop the phone into the holder.
I've been charging and playing audio from my phone for a few years now (HTC Mogul and then Palm Pre), and it's always a pain that you have to pull over, wait for a light or be distracted when driving just to connect the charging cable or audio jack.
I can definitely see how it's a little bit of waste to invest in something that will only last as long as the phone. I'm just tired of reaching down and fumbling for cables.
Proclip also has a Universal Mount at the bottom of the page I linked above.