Dedicated scanning ships, or ships equipped with dedicated scanners are built around that gameplay, which is not what the Mercury is built for. The Mercury's scanners are only good enough to intercept local communications, rather than scanning of unknown areas or picking up low signatures at distance like the Terrapin can. The Terrapin, when crewed effectively, will always surpass the Mercury in terms of scanning efficiency. Think of the Mercury as a hybrid cargo ship, between freight and data.