ERC20 tokens (along with tokens on EOS, tokens on Neo, and now even tokens on Steem via Steem Engine, etc, etc) aren't directly involved in the security of the system because they don't run the blockchain. That has been offloaded to the parent chain. The token, if it's a voting or governance token, may reintroduce a need for value to secure what is being governed, but I see it as a unique, case-by-case basis when it comes to tokens without their own chain.