My pet (slightly tin-foil hat) theory is that the draws to this kind of behavior is:
A) It's a status symbol of being able to afford a frequent 9.80$ expenditure on something as basic as frappé (or 5 for a normal coffee).
B) The typical sugar content of drinks (that have sugar/syrups in them) from places like Starbucks is addictive, like how fast-food chains add excessive sugar and fat their products (yes, there is a relatively high amount of sugar in their bread).
I really enjoy the occasional nice coffee with a friend or two, but a 9.80$ Starbucks frappé? To me, they often taste and look like some frakenstein of candy-floss, tiramisu, and ice-cream sundae.
One thing I would like to have some data on is what are frequent Starbucks customers drinking? Are the frequent customers predominantly ordering the addictive items, or is there no correlation?