Monero is a privacy coin that allows for multiple subaddresses.
The reason I prefer having identities (subaddresses) and to recover these addresses using a single 24 word seed phrase is for a simple one-time backup of your addresses. This will not backup anything else.
Lets say you generate a subaddress for every single friend you have and then a few years later your phone dies and you did not make a backup in over a year. The seed phrase will be able to recover your subaddresses. You may not know who address is for which friend, but as long your friend knows they can contact you and you will still receive the message.
If simplex implements multiple reuseable addresses that are generated by a seed phrase like a cryptocurrency, it will be a killer app. I am not against removing the single use one time codes to establish communications, that is a cool feature, just not possible to backup these communications on paper.
Paper backups are considered the most secure since it is not backup using a computer.