We used EZair for our last years and this years trip to London. The tickets were "issued" approx. 60 days prior. EZ air fares were comparable with AA premium economy fares. When we book AA flights separately, we always pay for the flight insurance, so it adds cost. When you use Princess, your princess insurance covers your flights ( Spoken from experience ) .
HOWEVER if you choose EZair way in advance and your flight tickets have not "ticketed" and you try to rebook a cheaper cruise fare your entire cruise may be rebooked including your flights, and you may have to rebook EZair at the current cost which will be higher in most cases.
As was said above, you can always cancel your EZair tickets prior to issuing and book separately at a cheaper fare if you find one.