Incentivized users are a very different user base than users found through other acquisition channels. In an internal analysis, the Google Research team found that incentivized users generally have lower retention rates and make fewer in-app purchases than users found through paid or organic acquisition channels.
To address those whose intent we perceive is to manipulate the placements of their apps, we will monitor and filter incentivized installs in our systems, including removal from the top charts. If warranted, identified apps also may be removed from the Store.- Google Play team.
So just like Facebook, create a fake metric (FB: Like :: Google Play: Download), let brands spend money on it and then bring the 'Don't be evil' mantra!