The advantage of Premiere Pro and After Effects together is that you don’t have to immediately render to get an idea of how an AE comp looks like in an NLE timeline. With FCP (or any other NLE) you have to render first, import into FCP and then see how it looks. If you need to make changes or add something, it requires another render.
Premiere Pro and After Effects share RAM preview, so can see how the comp looks like in context before you actually need to render.
It’s not perfect, but it’s definitely a nice little extra to have.
Dennis