"To what end? Familiarity with the Gospels is fairly useless apart from an intimate familiarity with the Old Testament."
Is it? I don't think so. Neither do all those missionaries out there teaching Christianity to folks who will never learn the Old Testament. Oh, I think that the Gospels are the key thing for Gentiles to read. Once they are understood, there is time to dive into the Old Testament.
What missionaries are they? I'm not familiar with missionaries that teach Christianity apart from its foundation.
I'm sure there are evangelists that look at conversions as notches on their Bible, leaving them to wilt after gaining a decision. Christ orders his people to make disciples, not converts.
You might think there is time, but that is a contrary to fact assertion as the Gospels cannot be understood apart from the OT. One can gain some insight by knowing the Pauline letters, but apart from the OT the Gospels are a familiar tune with unknown lyrics.