This sounds like a question of limited bandwidth. It doesn't really matter who is worth more or less unless there aren't enough employees there to take care of all the customers at any given time.
As one poster mentioned, if you're standing behind the counter pointing to a product, you could be walking the customer to the product, engaging in sales or small-talk, up-selling, etc. That's a benefit to the shop, unless it's taking your time away from a "big fish" purchaser. That's understandable, and is only rational retailer behavior, but the fact remains if you're standing behind the counter pointing, that necessarily means your time isn't being taken up by said big fish. So why not maximize the value of a small purchase whereas otherwise you'd just be standing there doing nothing?
As to the question of what value "regulars" are, beyond the obvious value of referrals, I can think of two things worth considering:
1) "Mid-level" regulars, perhaps myself included, have pretty much all they need to go out and enjoy a reef dive. We only spend a few dollars each time we come in. However, I've seen hardcore regulars walk in and leave with a few hundred worth of supplies (new valves for their stage bottles, replacement computers, dropping 3-4 regs off for annual service, etc.). It's not a new fish walking in and dropping $1,500 for a scuba package, but what kind of retailer would marginalize the value of hundred-dollar purchases? So I think part of me challenges the notion that "established" divers don't spend a lot.
2) Depending on how big the store is and how many regulars they have, even small-purchase-only regulars can contribute significantly to the "long tail" - somewhat counter-intuitively, a lot of people making a lot of small purchases can often result in higher revenue than relying on big-ticket purchases. I bet most LDS's are too small to benefit from this, but I bet large retailers like ST or LP take advantage of long tail economies, and would be very remiss to lose their "regular" customers over new buyers.