Is there any upside to terror?

There is a 1% random event that occurs when you're sailing with Terror ≥ 75 and with...

a tattoo. An Old Friend will give you the Lady in Lilac, a pretty good surgeon (+6 to Hearts, Pages and Mirrors) who also appears in somewhat different circumstances in Fallen London. Unfortunately she disappears when Terror drops below 25, so it's hard to keep her around.

Do remember that there are other, less pleasant random events that can also trigger when your terror reaches 75 and above.

You need Terror ≥ 1 to sleep in your lodgings, which has other benefits than reducing terror.

Most notable among these is reducing Menaces: Yearning, Burning by 2 to 3.

You also need Terror ≥ 70 (and Something Awaits You) to take the Listen action in the Iron Republic, which

gives you 70 fragments, plus one fragment for every two terror over 70.