Is Herculeum worth a half day?

I would say yes, it is very different than Pompeii. A couple hour visit is probably enough. Be aware that there is about a 10 to 15-minute walk between the train station and the ruins, somewhat downhill on the way, then uphill on the return as I remember.
I think your planning would depend on your interest in a Roman history and ruin sites, and the alternatives where you might spend your time. You could also add the ruins at Oplontis, near to Herculaneum, onto the same trip.
Personally, I would like to return to Pompeii and the Archeological Museum in Naples, even though I have been before.

I do it for a whole day, but that is because I go from Salerno. My museum/history brain doesn't do well for over 3 hours or so, so its small enough to learn a lot and appreciate the entire site. It had different conditions than Pompeii, so it is a different feel. But the question is what else would you be doing, and do you want to do that more?

Absolutely yes imo. Herculaneum is much better preserved and intact. Well worth a half or a full day.
There’s an exceptional Nova show, if you can find it, about Herculaneum which makes it all the more fascinating.