Welcome to SitePoint!
Having developed reservation systems for restaurants I can share my experience, whether that's applicable to your industry or not is up to you. We give merchants (restaurants) the ability to collect the postal address for two reasons:
- Some restaurants such as Melting Pot require a credit card guarantee with a reservation. The booking occupies such a large chunk of time that they can't chance having bogus reservations in their system. To get the best rate on a credit card authorization/sale, the postal address is used to return an AVS match.
- Most of the hospitality industry is still in the stone age when it comes to technology. These people are insanely busy and have very little time or interest in learning and dealing with technology. They're generally more interested in their own industry. When they think of marketing they can't imagine building a database without a postal address.
That being said, notice that I said we give the merchants the ability to collect a postal address, not mandate the workflow one way or another. Despite my generalizations many merchants want to collect the data, many want to collect it and require the fields to be completed and still others couldn't care less.
My recommendation is that you give the merchant as much flexibility as possible here. It's going to be much easier to create this flexibility in the initial development phases than it would be to retrofit the change request in live software.