Big companies that use patents as a revenue stream (MSFT, IBM, etc.) typically bide their time and bring a patent lawsuit once a new company is established and there is blood to drain. It's the threat of such a lawsuit in the future that can negatively impact investment in a startup, as the right collection of patents could conceivably capture much of the economic surplus of a new venture. Alternatively, a big company might use the threat of a patent lawsuit, now or in the future, to push a young company to agree to an early acquisition.
The pledge doesn't seem to have much impact on these scenarios, even if a big company were to follow it rigorously.
Most of us, I believe, would prefer to see companies make a stronger commitment: "No first use of software patents" [period]. Google hasn't made this pledge, but to the best of my knowledge, they've acted in this way so far. It does seem in line with "don't be evil."
That said, I think I see what PG is going for here. He wants companies to make a pledge that, at a minimum, allows a new product or service to be tested on the market. That way, if it gathers traction, it will attract investment despite the threat of patents, and the new company will be able to mount a reasonable defense.
Perhaps more importantly, though, by allowing the product to succeed first, even in a modest way, it makes the offensive use of patents worse PR for the big company. Killing a successful product with patents is no longer an abstract issue. It takes away from customers and the market something very real.