Lets say a restaurant closes at 10pm.
And lets say you have a 10pm PS.
Usually you will be allowed to arrive up to 15 minutes "late" and they should still honor your PS.
BUT, if you did not have a PS, and you came up to the podium after 10pm, they do have the right to turn you away.
Now, before you say, "well that's not very Disney-like" consider this.
The serving staff is geared towards how many PS are on the books. On a slow night they might "run" 10 servers, while on a very busy night they might "run" 13 servers. Some slow nights, parts of the restaurant may be "closed" because there is not a server in that station. So, as the shift moves on, the staff will close servers in anticipation of 10pm and also based on the remaining PS in the system. At 10pm, there may be only one or two servers left on the floor, and even though there might be lots of empty tables, it is possible that there are no servers that can handle those tables.
So, that is why I advocate making PS arrangements, even in slow times, it affects more that most people think.