Dreams Riviera Cancun (and the NOW resorts, Azul Sensatori) are SUPER popular and close out (meaning rooms are sold out) really early. Sometimes 8-9 months early. We even have closeout dates for 2015 already!
also, the rates you are locking in are most likely much lower than what will become available later on. AMResorts is very consistent with their pricing so it usually goes up as you get closer to travel dates.
Also we have a pretty close relationship with them so we are able to reduce rooms if you find that you no longer need the block. Better safe than sorry because we have had many brides on here not have rooms for their guests.
Regarding air rates, usually once you book air, you are locked in whether you book in a group or as a package so I am not sure what that has to do with your decision? Air rates are controlled by the airlines.
Additionally, you can wait to see what the air + hotel package rates are but there is no guarantee that this rate will stay unless you block your air which is difficult and not usually cost effective. The package rate can change within the hour and usually they only have a set amount of packages at that rate so it isn't like you will have 20 people book at that rate.