A Safe Harbor means the IRS can not question that it qualifies as a Trade or Business.
You are signing the tax return under perjury anyways, so signing the Safer Harbor doesn't really change anything in that regards.
There are also court cases that have rules a bit more strictly. Personally, I think anybody would be foolish if they meet the Safe Harbor and don't use it. I would rather have the "Trade or Business" affirmed with no-questions-asked rather than arguing with an auditor and going to court.
As a further note, in past years many landlords have NOT been sending out 1099-MISCs for services to non-corporate contractors. By doing so, they have been implying that the rental is NOT a Trade or Business (because if it was a Trade or Business, 1099-MISCs are required). If I was an IRS auditor, that circumstance would certainly make me consider denying the QBI because the taxpayer has not been treating it as a Trade or Business. If the Safe Harbor was there, the auditor could not do that.
