Actually there is a tangible economic incentive not to use the free down votes:

retaliation!

I used to run a daily script that processed all flags done on one week old posts, until Steemit Inc killed my code by implementing API rate limiting a few months back (here is a sample graph of one day. One thing that stood out was that the same accounts kept showing up in different roles, either as flagger or flagee. On closer inspection, a substantial amount of down vote weight could be attributed to some form of retribution for earlier flags against accounts with likely relations (voting proxy or non-standard recovery account) with the flagging account.