We flew MAN/EWR a couple of weeks back and the seatbelt sign was on the whole flight. In the end I grabbed a passing crew member to ask could I please get up and use the loo. He said sure, just be careful. There were 2 little wobbles the whole flight. I got up again later to see about getting a cup of water and they had trays of them laid out in the galley, so obviously CO expect the pax to get up and help themselves, but leave the seatbelt sign on. Curious.
This was a single aisle aircraft too. So wondered if that may have played some part in leaving the sign on. It also carried only 4 crew, so guessing easier for them to deal with strapped in pax in the unlikely event, etc. The chap serving our area was really, really nice and kept checking we were ok.
We always strap our seatbelts regardless, we once saw the effects of CAT over california and it wasn't nice.