Interesting points. As far as I can tell, offering promotions has three possible benefits:

1. Introducing people to your product that might not have otherwise tried it, or might have forgotten about it. Some of these people may become loyal customers in the future paying full price.

2. Price discrimination: those with lower price sensitivity won't bother with the coupon and will pay full price, while those with higher price sensitivity will still be your customers. This only makes sense if you are making money even at the reduced price.

3. Creating a customer perception that they are getting a better value.

None of these things really works if everyone has ultra-easy access to coupons all of the time.