If the customer wanted to deploy multiple SSIDs, they would have to configure every switchport that an Instant AP is connected to with a trunk. If the customer is deploying alot of APs, it might be easier to have a controller in that situation, because the only place a trunk would need to be configured is the uplink to a controller.
If the customer has multiple remote sites, the customer would deploy instant, to avoid putting a controller at each site.
Since it is a shopping mall, the customer could deploy a controller to save the task of configuring physical ports as trunks connected as APs. If the customer does not mind configuring trunks connected to each AP, they can go with instant.
That is what it comes down do in this specific situation.