The password is used for encryption. I think it's important to keep safe the file that has your "real" passwords.
Re: AquaMail does not, in any way, need to decide for me what is safe and what isn't.
Personally I don't want complaints that "this app is unsafe, is saved my password in a plain text file".
Second, isn't it an inconvenience that e.g. mail services require a password?
"Why does Gmail require me to enter a password, I just want to access my email".
Third, not being able to restore from backup - does not prevent you from using the app (yes it will be necessary to re-add the accounts and re-enter your settings).
In short - it's a security / convenience tradeoff, and given that "restore from backup" isn't a feature one needs daily, and that the protected information is rather sensitive, I personally think that the bias towards security is entirely appropriate.