It is indeed a brilliant marketing tool, but it's one that does a huge disservice to both students and dive shops.
The beauty of attending classes at the dive shop is that it gives students a chance to interact with staff at the LDS. In this day and age of dive shops struggling to compete with internet merchants, the importance of this can't be overstated. If the LDS is just a 'place' where folk go to do their pool skills - especially since it's entirely possible they'll do their open-water dives someplace else, such as a tropical dive destination - then it doesn't give LDS instructors and staff much of a chance to form a relationship with their students/future customers.
Attending lectures at the LDS also gives students a chance to socialize together which is (lets face it) one of the best things about diving. It's pretty hard to socialize in the privacy of one's own home, doing an online e-learning course. More fellow divers as friends = stronger likelihood of staying with the sport = more $ for the LDS.
The way I see it is quite the contrary. As an instructor I would love for students to come to class prepared so we could focus our energy on the diving instead of talking about diving.....
A typical OW course at the shop I work for includes something on the order of 12 hours of theory. If we could cut that back to 2 hours because the students were coming prepared and spend 6 hours on an extra day of diving then we'd still be done faster with the benefit that the students would have more time in open water for potentially lower cost. (or for the same cost but the LDS get's a slightly bigger margin).
As for selling stuff and client binding. My thinking is that even if they do the theory at home they're still going to need gear to go diving and as far as the social aspects and networking goes, making that work is more a matter of how you organise things than anything else. For example, nothing stops you from organizing a "club" of recently certified divers. You could give them discounts on gear and get them coming to the shop once or twice a month to go diving with a group of other divers with similar experience levels.... The shop I work for does this and the way it looks to me the "club" does as much (or more) for client binding than trying to survive listening to an instructor talk at you for 12 hours....
R..