Why not include a complete list though, so long as it can be maintained?

If the information is public I don't see any good reason why it must be a subset.

I think that is what the VC company's site could be used for.

I don't. Lists are easily maintained on a wiki with a large number of users, which you wouldn't get with a VC company site. Given that the wikipedia article is about a notable company, and the list of funded companies is one of the most important facts about that notable company, it should be included in its entirety. There is almost nothing to be gained by having a partial list.

I would think that either you'd include a list that is as complete as possible, or not include it at all. Doing a partial list because you don't WANT it complete is just warped.

By that logic, ANY entities site could be used for ANY information on Wikipedia. The point is that it's valuable and useful information that isn't readily available (most VCs keep incomplete lists, probably to marginalize the failures). The lists would enhance the knowledge of the reader, which SHOULD be the goal of Wikipedia.