On my laptop (MacBook Pro mid-2010), Spotify (18.104.22.168.g19de0aa6) always uses the discrete graphic card (both the application itself as well as the helper deamon). Consequently, Spotify drains the battery.
After investigating the issue, I found that there were more users experiencing the problem, which was fixed using the following: https://developer.apple.com/library/content/qa/qa1734/_index.html, which Spotify has now adapted (which I found when I looked at Info.plist). However, this problem only fixes the issue for 2011 and later MBPs. Other's suggested it was due to the advertisements shown in Spotify, but I have been using premium for years now, so that is not the issue here.
Therefore, I would like to submit this as a new issue (or old issue that has not yet been fixed).
We're closing this thread as we haven't heard back from you in a while. Don't hesitate to give us a shout if you still have the same issue.