What advantage would Tesla gain picking up all the dead weight that GM has, with its dealerships, unions, pensions, and huge factories dedicated to building ICE's?
Most of what would be left is the brand (mostly worthless, except stuff like Chevy trucks and Camero), and the engineering / IP. That might let them expand into more body styles more quickly.
They could also take over GMC’s fleet contracts, and the financing divisions loan servicing. Those are pretty lucrative.
Having said that, I doubt it makes sense, or would go well. Union busting and stealing pensions would severely damage the Tesla brand, and I don’t think they could do any of that stuff outside bankruptcy proceedings. If they could, GM would have already done much of it.
Also, trying to integrate a car company with a completely different business model and tons of legacy baggage is probably riskier than continuing down the path Tesla is on.
Is this positive or negative value for Tesla? A lot of a Tesla's value is it's distinctive look as a status symbol, meanwhile Tesla is primarily production constrained in how many they sell. Expanding into more styles lowers the value proposition of buying a tesla, and they probably can't actually produce enough cars to take advantage of the moderately larger market.