Yes , most Sun resellers are indeed useless at marketing, and don't realise how difficult they make it for potential customers to find them. That's because if they aren't listed in this directory, then they are even less likely to be listed anywhere else. That's not too diificult, and companies get added typically within 1 or 2 days. If there were only a handful of Sun resellers in the market, the market would be very unhealthy. You wouldn't need to come to this directory to find competitive suppliers, and I would need to find a new job.
Manufacturers have diverse policies about publishing lists if their own resellers.
Some do make these publicly available. Sun Microsystems seems to have a policy which varies from country to country. But Sun is not in the business of helping its competitors, so even where they do make this kind of information available, it typically excludes a significant number of companies which sell compatible products made by competing oem's. Another reason that manufacturers don't like publishing reseller lists, is because that makes it easy for their own competitors to target and poach their best customers. Feedback from that quarter would suggest that resellers in all segments of the computer market are equally good or bad at marketing, so perhaps I'm being unfair at focusing on the Sun segment.