Agreed. Personally, whenever I study or do work in a coffee shop I try to regularly make purchases.
I usually end up averaging between $1 and $2 per hour which I think is completely reasonable for the space, drinks, snacks, and internet access. I'll ramp it up a bit if they get busy and if they stay busy for very long (30 min. or longer) I will usually leave.
$1.50 espresso for 3-6 hours feels a little like you are taking advantage of the coffee shop. There is always going to be some variation, but I would prefer to be a customer that helps a business--that provides a service that I like--stay profitable rather than one that helps put them out of business.
If/when times are hard it is good that you can do this, but otherwise if you can afford to support a business that is supporting you then order the cappuccino!