Super interesting idea. One of the cons could be that having your employees not personally invested in the outcome of your venture could make them less likely to be "fully committed" (i.e. long hours, going above and beyond). You might be worried about higher levels of apathy -- "if this doesn't work I still have a job".
If the business works, I'm thinking the transition from the fund swat team to "real" employees might be difficult as well, probably at a time when you need to be firing on all cylinders.
My thought on the "if the business works" case is that such employees should have a clean path to either transition to the company they helped make a success, or get reassigned by YC. Either one is a win for YC in this scenario: An employee which helped a startup succeed is a hot commodity they'd love to reuse, but they also own a portion of the successful startup, so having it continue to succeed is also a big win.
The idea here would not be for YC to have a constantly growing outsourcing team, but to ensure they didn't lose the good employees because a startup failed. In a given failed startup, there are likely some rockstar employees that definitely weren't why it failed, and it's in YC's best interest, presumably, to hang onto those great employees and get them placed with other investments of theirs, should the opportunity arise.
> "fully committed" (i.e. long hours, going above and beyond).
Regular comp does that if the company can afford it. Startup equity is there so companies that can't, can offer early hires the "maybe money". Pay 200k and you'll get fully committed employees.
If the business works, I'm thinking the transition from the fund swat team to "real" employees might be difficult as well, probably at a time when you need to be firing on all cylinders.