Actually, the reason I gave is merely a cluster of special cases of the "insufficient unique content" rule.
Here's how.
Promotional content doesn't count. Like ad banners on a page, we ignore it (if we can) on our way to the INFORMATIONAL content. Some verbiage serves both purposes -- and counts as informational. Obviously you're thinking in terms of promotion, so this last represents the only possible intersection of interest. But there's nothing preventing you from having some promotional content on a site listed in the ODP because of its rich informational content.
Here, we're concerned with information about a company. And that is either authoritative (that is, from the company itself) or independent (that is, "consumer information".) Authoritative information can be spotted by its being linked with approval from the company's home page (whether or not it's on the same domain), so we only need to give one URL.
Now, your content is either promotional (in which case we do not list it) authoritative (in which case it is the company's responsibility to link to it, and we just link to them), not unique (in which case we list the more authoritative company site) or not authoritative -- even the company itself didn't link to it, in which case it can be assumed to have very low informational value.
So it's either not unique or not information or not of interest even to the most interested party. We don't necessarily know which, but it doesn't really matter. It's easier to say "ah, we've already got this company listed; it's their responsibility -- not ours -- to have all their information linked together."
That gives us a simple guideline that fits most cases: one company, one website. It rewards well-interlinked sites. It places the responsibility for presenting corporate information on the corporation where it originates and where it is most valuable. It protects us from the abusers who say, "My company website doesn't link to all its pages, so you guys have to" or "I crippled my own website navigation in order that you should reward me with more deeplinks." (and perverse as that seems, it is not all that unusual a demand).