On the other hand, if a civilization limited to centripetal-gravity starships were to live next door to folks who don't have a Prime Directive, it's unlikely they would get to launch those starships before the neighbors came a-knocking...
We don't know if Earth ever sent sublight ships to other stars; there would certainly have been the opportunity to do so between the 1990s and the 2050s, with at least one ship demonstrably (even if clandestinely) going interstellar as seen in "Space Seed". However, Earth at that point already had artificial gravity down pat (even thought that Mars ferry in "One Small Step" had its gravity turned off for some reason when the camera visited it). Apparently, inventing AG is easy and cheap, and everybody can do it before going interstellar. Perhaps AG is also a required intermediate invention in the path of developing an interstellar engine, meaning nobody ever builds a starship without AG?
Timo Saloniemi