For one, I want my music on the phone because I travel a lot and don't have network access all the time. Google Play Music can store the music on the phone by "pinning" it via Keep on device, but it stores in some obscure location on the phone as do many of the other "music managers". The beauty of iSyncr is that it stores your music in the Music folder and keeps the folder structure exactly as you would if you did a straight copy. The added benefit is that I get my playlists copied over as well, and I don't have to do anything different on my pc or learn a new music manager. iTunes is just fine. I tried MediaMonkey and other similar ones and they are no better than iTunes and in many cases not as nice. I think people criticize iTunes because they read that its bad, not because they tried it.
Understand, this works for me. For those of you who don't have iAnything, this may not be ideal. But, I believe there are plenty of iOS/Android setups and keeping both platforms satisfied is necessary.