They are entitled to order any any aircraft they want, I’m pointing out that I don’t see any “hot high” advantage to the a319neo vs the a320neo. Yes, I fly the aircraft, I know it’s capabilities, the a319 had some advantages over the a320 into certain unique airports, but the a320neo isn’t the a320. It’s far more capable, and I don’t see an example of a route where the a319neo performance would be warranted. Maybe it is and I just don’t see it.
I think what needs to be kept in mind is that a ULCC like Spirit will operate their fleet much closer to the aircrafts MTOW because of their higher seating densities. Small differences in range and takeoff performance make a big difference when closer to the aircrafts MTOWs for reasons like higher seating densities. With higher seating densities an airline will get to the point where they have to trade payload for fuel (range). For example, I’m sure AA would have no problem launching a full A320neo from BOG to ORD or B6 launching one from BUR to BOS but I’m not so sure that Spirit could do the same with a full A320neo based on their increased payload due to their higher seat density. These routes may be possible on Spirit, though, in a full A319neo.
Range numbers and takeoff distance data that manufacturers advertise are generally based on a typical airlines (like AA or LH) seating density and not a ULCC like NK or F9. With higher seat densities, range will be lower and takeoff runs/stage 2 climbs will be longer.
Notice there is no mention of the 321LR being ordered for Spirit. The 321LR With simply longer range due to added fuel tanks and no increase to MTOW would make no sense for Spirit as they would have to trade payload for fuel due to their higher seat density. Other airlines wont typically have to make this tradeoff due to lower seating densities. Now the 321XLR may be a different story for Spirit in the future as it is getting an MTOW increase.
Long story short, where a 320neo and 319neo may be very similarly capable for an airline with a standard seating density, the performance differences may be notable for one with a higher seating density, thus the A319neo order from Spirit.