Search Partners is on by default in most Search campaigns, and the usual case for keeping it is cheap extra volume. My concern is what it does to bidding. Smart Bidding optimises toward a single target, so if partner clicks convert at a fraction of the rate of Google's own results, the tCPA gets dragged toward the weaker traffic and the blended number hides it.
Take a plumber getting 40 leads a month from Google and 10 from partners at twice the cost per lead. The total looks acceptable, but the partner share is the reason the account costs more than it should. Segmenting by network in the Segments view shows this quickly. I'd only keep partners if the split shows lead quality holding up, not just cost per lead. Has anyone run that split and kept them on?