I think it depends on the restaurant. One time, we had a reservation at Crystal Palace. My brother wasn't there, but, the rest of us were (a total party of 8). They made us wait until he was there, and then ,we waited about an hour longer, because, by then, there was no table that size to seat us. And, even at that, we were seated at two separate adjacent tables.
Then, another time at Boma, we made a reservation for 5, but, two weren't able to make it. They seated us immediately, and were totally cool about the situation.