MiGs give some ability to handle cultist aircraft, but nothing really dominant, and Interceptors/Ravens can come online so soon afterwards that it's really not worth it unless you're specifically focusing your research on other topics and trying to make do in the meanwhile. And even Interceptors are so much worse than Ravens for actually chasing UFOs and not too far research-wise, so it's better to get Ravens. Thunderstorms and weapons for them can take a bit more time, so at least a few Ravens are good if you do play the air game. Which, as noted, isn't truly necessary.