They used to do stuff like this all the time in 'Buffy, so I had no problem with the show going into the spiritual...
To each one's own, of course, but to me, the difference is that Buffy was a fantasy from the start, so supernatural things fit right in. But Orphan Black is pretty much a hard science fiction show, one of the very few, so injecting mysticism would be a conceptual and stylistic mismatch. There are certainly plenty of shows that treat SF and fantasy as overlapping and interchangeable, but that's just why I like it on those frustratingly rare occasions where a show sticks to hard science, and why it's even more frustrating when a show that was one of my few oases of science in a landscape of fantasy ends up swerving into the paranormal.
So I take solace in the idea that the scene can be interpreted merely as Sarah's dream/hallucination. It's not actually Beth's ghost talking to her, it's her mental manifestation of her guilt about Beth. Which is why the dream happened in Siobhan's kitchen rather than someplace from Beth's life -- because it's tangled up in Sarah's memories and associations. (Also because it's cheaper to use a standing set than to rebuild Beth's apartment set.)
I love how Dyad is kind of back in the picture now, what with their absence from most of the season. I really like how they're integrating Rachel into the story line. Could she maybe become a part of clone club in Season 4? I know it seems implausible now, but I never thought Helena would be dancing with the rest of her sisters in Season 2 when we first met her in Season 1.
That's an excellent point. Helena went from Sarah's most feared enemy to someone Sarah cherished and wanted to take care of. Maybe Sarah and the others can find a way to embrace Rachel, now that she's so vulnerable.