As far as giving out freebies goes, what hasn't been explicitly pointed out is that there is more than one type of customer, and each type has different value to the store and will respond differently to good service. They have all been used in examples so far in this thread. There may be more, but these are the ones I can think of:
Type 1: the cheapskate. Wants everything free or as cheaply as possible. May never show up in your store again, little or no sense of loyalty. Doing favors for these people will probably get you nowhere, since they come in expecting it from you.
Type 2: the newbie. Doesn't have any loyalties yet. Maybe they just started diving, maybe they just moved into the area. They will easily develop loyalties if someone is consistently nice to him/her. I think people are most interested in making new connections when in this situation, so if you give this person a few freebies, they may very quickly become loyal to your store. It happened to me both with my dive store and with my mechanic, and there are many other testimonials that have already been mentioned here.
Type 3: the disciple. Loyal to your store. There seems to be agreement that being nice to this person is a good idea.
Type 4: the enemy. Already loyal to another store, but happened to enter your store with a specific agenda (e.g., need to buy XXX piece of gear that their own store doesn't sell, or maybe their store is too inconveniently located). I have been guilty of being this person many times, but I never expect anything above and beyond being sold what I'm after. By doing favors for this person there are possible advantages, but don't expect to add this person to your customer base. It may be possible to sell this person something which your store carries and their store does not. They may also come back a year or two later when they need something else, or recommend your store to others (after thier own, of course).
Type 1 I think is rare, especially in this extreme form that I described. Remember, the customer already spent money and time to get to your store and walk in; you might still be able to sell them on an accessory for their regulator you just assembled (retractor, spare mouthpiece, hose wrap, etc.).
There are probable, positive consequences of spending 2 minutes of your time to help out types 2, 3 and 4. If you don't treat the customer well, you'll lose him or her regardless of type (but that hypothetical extreme type 1 probably has nothing to offer anyway). The trick is, though, how do you know what type of customer just walked in your door? Sometimes you don't know, so when in doubt be nice. It costs you very little, should make you feel good at the end of the day, and it has the potential to benefit your business. Of course if the same deadbeat person keeps coming in, not wating to buy anything, and expecting free favors, you should just tell them to get lost.
Ben