I think it can be a selling point in the case of exchanges where there's strong contention about character or pairing assignments.

Teen Wolf, for example, has pretty much always been dominated by the Sterek (Stiles/Derek) pairing, which those of us not into are really not into. I'm reminded of one exchange where I was matched on a rarepair, but my recip's letter outright stated that they assumed Sterek would be the primary pairing in the resulting story. Hand matching could have avoided this. As could have better modding, I suppose. At any rate, it was unpleasant for me.

I don't know if FK fans have such hard-drawn lines in the sand, but certainly mentioning that slates are hand-matched could be consoling to people who do have reservations.