Insofar as financial instruments are fungible/transferable, they ought to be transparently so. OnIt's not about the financial instruments. I'm making the assumption that a "side project" that the spouse did with a few of his friends wasn't capital intensive, and it was knowledge based. As a software developer, if I were to start a business, it wouldn't be capital intensive at all. I would need partners that brought a set of skills to the table - maybe a few developers and a person with a marketing background or industry connections. The only costs besides time would be a bunch of AWS resources. The "investments" in my company wouldn't be financial, they would be expertise. Why would one assume that the person's spouse would have the expertise to make intelligent strategic decisions on the direction of the company? What if it were a law firm?
Do you think that Carl Icahn or Baine Capital have the same set of incentives as the employees or the founders of a company? They want to make their money and get out, their time horizon is a lot shorter.
The same could be said of the spouse of a founder. As founders, we would be more likely to care about the long term vision of the company. The spouse may just want to make any money she can right now.
If Mark Zuckerburg hadn't kept a controlling interest in Facebook, would it be where it is today? What if he had sold a controlling interest to Yahoo under pressure from the VCs?