What risks are involved if a new joiner doesn’t check signatures while downloading the blockchain?

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Clearly once you’ve downloaded the blockchain and started mining, you must check signatures otherwise others could spend coins without private keys. However, as a new joiner joining the network, what benefit is there to checking signatures? If a new joiner accepts blocks buried under thousands of miner confirmations, without checking signatures, what could go wrong? The only concern I have identified is creating additional motivations for 51% attackers. Is this a big deal?