Its really a recommended feature on the free plan, so we can purge for example https://blog.example.com instead of the entire zone. And still Cloudflare insist us to not purge everything in a zone…
Reminder: We highly recommend performing single-file purges.
It’s not a big deal to Cloudflare to do a Purge Everything. It just might be overkill for what some people would want.
If you want Purge By Host, that’s an Enterprise feature.
Then it might not be no big deal to Purge by Host.
On an Enterprise plan it isnt a big deal 
You generally seem to request a lot of features which are on paid plans for the free one. I am not sure this is the overall intention of the different plans and I guess if Cloudflare would have wanted to offer them on the free plan, they’d be already on the free plan 
Wish I could afford one 