Most retail Aeroplan partnerships generate very few points outside the occasional promo. The expectation isn't that any customer is going to earn enough points from this one partnership to cover all their travel needs or anything like that, but they want to give you options so that you keep using the program and racking up a few miles here and there. They're not hoping that people start spending thousands of dollars at Starbucks just because they can earn Aeroplan miles, it's about influencing a few decisions on the margin - maybe you buy an extra something from time to time, or more importantly you choose Starbucks over Second Cup or Tim Horton's or whatever.

There may also be a data aspect to the partnership, from the Starbucks perspective. Obviously Starbucks can track what their own customers are spending at Starbucks, when they use the Starbucks app, but this partnership may buy them access to info from Aeroplan credit cards that would give them insight in to where else their customers are getting their coffee/etc and opportunities to target those folks (and may get a few more people to sign up for Starbucks' own program who previously didn't bother).