"Having someone walk around asking for orders takes more time away from making orders, cleaning the equipment, grinding the coffee, etc."
But coffee shops already have people walking around a coffee shop anyway, so they might as well have them try to extract more money from customers (and more importantly, poke the deadbeats).
I doubt it's all that different from Nandos - a chicken restaurant chain that's virtually non-existent in the US but very popular in the UK, where you order you meal at the counter but staff come around asking if you want dessert/coffee when you've finished. Seeing as Nandos generally have free wifi and always have bottomless drinks and frozen yoghurt, people could conceivably hang out in a Nandos like they do in a Starbucks - but people generally don't and I bet having staff poke people is one reason.
Also, I doubt staff would be bothering people every 15 minutes - it would be the same frequency that they already bother people while collecting empty cups. While it's not without some cost to the shop it still has the potential to make them more money, especially if they mostly focus on people who are on their own.
I think the real problem is that the standard coffee shop model is that it works best for small groups of Friends who come in to chat over hot drinks. Unfortunately, on a week day they're more likely to get business from "loners with laptops" and "business meetings". While the model works ok for the business meetings, it fails dismally for the loner with a laptop. Getting rid of WiFi is one way of solving the loner with a laptop problem, but it's difficult to see how it's the most profitable way of solving the problem and it doesn't solve the loner with a book problem or the pensioner with nowhere else to go problem.
