I can't speak for Kochuveli express as I've never travelled by it. But as far as Seshadri and Nagercoil express are concerned, they have very good patronage from SBC itself. In fact Nagercoil express was introduced on the demand of the locals only. And besides, if the RSA between Seshadri and Nagercoil is not removed, then no matter where they terminate, each rake will still have to be parked for a few hours. ( Nagercoil express is only parked for 2 hours at SBC before it departs at 11 AM as Seshadri. 2 hours is a very reasonable parking time and does not restrict SBC in anyway).
If rake parking is unavoidable, SBC is better than BNC as...
more... BNC has only 2PFs for passing trains and only 1 empty lane for parking. SBC at least has multiple platforms that can be used for parking the rake. But best solution is to remove RSA between Seshadri and Nagercoil. That way Seshadri can terminate at SBC itself without occupying platform for a long time.