The most compelling of reasons for airline companies to pay their staff during training and also for type rating is the quality of product. An organisation that renumerates pilots from the commencment of employment ensures that there are minimal financial distractions during a time when full focus is required to learn and pass a check.
Companies that don't pay pilots during training (and for type rating) telegraph the message that the welfare of their staff is secondary to profit. However, it is also a simple minded approach because as a result, the quality of the training is potentially degraded if there are financial worries held by the trainees. Low quality training = low quality operation.
Simply from a resource management perspective it makes sense to pay and pay well from Day 1. This approach would at least ensure that trainees are afforded a minimum of stress and distraction at a critical time and also would serve to attract a higher calibre of candidate.