You suspect that they must have reasons behind their design and feature choices but there is always people who are left disappointed with their choices. Some fear this could start a trend and encourage other streaming services to remove their Android homescreen widgets, but given the backlash Spotify has faced, that probably won't happen any time soon.
Spotify have said on their community page that they are "retiring" the widget and putting their "energy into new ways of creating the best experience" for their users. Spotify users will still be able to access the same playback features and information about what is playing through the app's playback notification and device lockscreen.
After what must have been quite a few complaints a moderator there has suggested folks vote for the idea to reinstate the widget. There was a widget for task managers, a widget for Google+, and even widgets for Google Currents. Regardless, Spotify is purging it from the app this week. Spotify has marked the idea "Not right now", but it's watching the numbers. However, it's not like the widget needed a lot of upkeep-it was a pretty generic resizeable playback control with album art. Though homescreen widgets are one of the unique features Android offers, Spotify has decided that it no longer wants to support its own widget.