Priority Seatings are not the same thing as Reservations. Therefore, no tables are being held for you and the restaurants will not lose money if you don't show up. The PS is used just as a way to sort of organize the guests who arrive to eat, and if there happen to be tons of people waiting to eat, having a PS will get you in before the folks who do not have a PS.
Technically, none of the restaurants in DLR (and that includes both parks, all 3 hotels and DTD) actually require a PS be made, except for the Blue Bayou (though there have been some reports that folks have occasionally walked up to BB and gotten seated on the spot, early in the day), and for that you should probably make a PS well in advance if possible because it is extremely popular. You can generally just walk up without a PS and give your name at any restaurant, but you may have to wait a long time to be seated. We used to always go to DLR without a PS because we wanted to go with the flow, but after waiting and wasting too much time at too many different restaurants, we switched to making PSs and have never looked back! It is such a time saver.
The restaurants to watch out for in terms of big crowds and long lines are: Rainforest Cafe in DTD (especially for dinner, it can get crazy busy and you could wait as long as an hour for a table); Carnation Cafe in DL (great little restaurant with very limited seating - I have never been there when there has not been a long line formed with people waiting for tables); ESPN Zone in DTD (just like RFC, very popular and the wait to be seated can be ages on a busy night), and of course, any character meals throughout DLR are usually quite busy and the wait could be long. Those are the restaurants that I have encountered very long waits at, so I would always make a PS for those from here on out. Other places we have eaten in DLR did not seem to be too bad as far as wait time.