If there was no monetary incentive for Rogers, they wouldn't be doing it. It's that simple. Why would they do that to their customers, creating all that ill will, unless they made money by doing it? I was born at night, but not last night!
The problem is that people are so used to Telcos doing this sort of thing "from time to time" as you aptly put it, that they tend to shrug it off, and the Telcos know that. Can you imagine a retail business treating a customer like that? I don't think so. Can you imagine having , say, a gift card balance and the retailer decided that they would only honour it up to $150, even though the balance was $300? I actually think there is a law against that in Canada, if my memory serves me right. That's exactly what Rogers is doing to my wife. Of course they do that "from time to time" don't they?
Gift cards actually did have expiry dates, which upset a lot of people who mislaid them or forgot to use them in the allowed time. That's when companies were forced to do away with expiry dates. Unfortunately, prepaid phone users are a small minority with no voice.