By having this API parameter we can run visual diffs against production wikis as templates, etc. get fixed and that gives us a pulse for how fixup is going and when Tidy can be realistically switched off.
There are two other alternatives: (a) use Parsoid's visual diffing as a proxy since the same HTML5 issues exist for both Parsoid and RemexHTML (b) periodically sync the dbs on mw-expts and mw-base VMs.
(a) is simple but results can be a little less authoritative. (b) is cumbersome and labour intensive. So, adding the API parameter would be ideal. Running visual diffs against the production wikis also lets us do targeted outreach to editors for fixups based on those test results.