It makes sense for crew issues. If you split the Airbus & Boeing between all three hubs, then it could be that due to illness, or unplanned last minute changes that you only have say airbus crew in ARN when you need Boeing (or vice versa). This might be better for smaller hubs.

Also, having different suppliers allow you to do a couple of things;
1) It keeps them competitive against each other, otherwise the prices can be kept higher. (When BA applied for a new short haul flight in the 90's then Airbus didn't want to answer fully as they saw BA as a Boeing only supplier and had been messed about by them before. BA had to reassure them that they were serious in considering Airbus, and did actually buy planes from them in the end).

2) It gives you more flexiblility. Different planes in different configs and different capacities allows you to change things around easier and last minute if required, especially if a plane goes tech and you need more seats on a route for the rest of the day for example.