One of the things PG reiterates is that at the early stage what the startup is doing changes rapidly. Many companies that end up being very successful have a product that looks quite different from what they set out to achieve. In such a setup, half-life of a business plan may be measured in weeks and its rigorous analysis may be a wasted time.
Instead, I believe YC is trying to focus on finding good teams under the assumption that a strong team that can execute well has a higher chance of success than a worse team with a better starting plan. If anything, the business plan's value is to indicate that the founding team can think logically and realistically map products to market. Once the investment reaches 100M (YC Continuity's stated higher end), they probably focus more on the marked and business plans. Just my 2c.
Instead, I believe YC is trying to focus on finding good teams under the assumption that a strong team that can execute well has a higher chance of success than a worse team with a better starting plan. If anything, the business plan's value is to indicate that the founding team can think logically and realistically map products to market. Once the investment reaches 100M (YC Continuity's stated higher end), they probably focus more on the marked and business plans. Just my 2c.