There's a fairly mad theory that planet 9 (which probably does not exist) is a primordial black hole, in this case meaning an object far too light to have formed from a collapsed star. I suspect primordial black holes may not exist, and we've already done measurements which show they are at least scarce if they do. Well, here's an experiment which will tell us if any exist in the outer Solar system. In the likely case that it finds none we have another data point which puts an upper bound on how many there are, which is worth having. In the unlikely case that it does find one or more then not only do we know black holes which are either primordial or have some completely unexplained origin exist, we also know of a black hole which we probably could send a probe to without implausible engineering. We could send a probe which does direct measurements of the immediate environs of a black hole. Nobel prizes fall like rain on the people involved and we're suddenly living in a world which is as cool as fuck.
I'll take 'small chance of discovery which will change physics for ever' any day, thanks.