All the skill cards have been positive to helping the investigators. How is this particular card a benefit to the investigator?

At first blush, it does seem more of a detriment ~ but really, it's one free Clue token, every round!

I personally love that card. Sure the -1 san hurts, but it's now only four tokens to seal a gate, and +1 bonus die on a check once a round. I always am happy when I get it... unless I'm already a 7 stamina / 3 sanity character... then it kind of hurts. Especially if you're against Cthulhu.

Had that once: Mark Harrigan, Cthulhu, Mythos Lore. I joked that "at least he's at maximum stamina all the time."

Of course, against revised Cthulhu, he won't last long at all.