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

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?