GrapheneOS Sharing the same exit IP address across multiple of your devices as opposite to having it finer-grained than a whole device isn't desirable.
This is only true if you take a very narrow view of its intended use. It is extremely desirable if you consider the most likely use-case, simultaneously running a VPN and firewall app, which, in so many words, may very well be the most requested feature and recurrent topic on this forum. A toggle of sorts that allowed users to share their Owner VPN connection with, at least, a sub-profile would be perfect for this.
That toggle doesn't exist, so it's currently impossible to run the more privacy-respecting VPN apps like Proton, IVPN, or Mullvad simultaneously with something like Blokada or Adguard because both are afforded only a single 'slot'. People now have to either root the device, forfeit one option, or mess around with not the most intuitive hack that allows a firewall app to offer some VPN functionality. The complex but otherwise useful RethinkDNS is commonly recommended for that last option but the app is really a DNS/firewall app first and 'supports VPN' second. The UI is not the most intuitive thing to someone used to the earlier mentioned VPN apps, causing the mental burden of worrying about pressing any button because the wrong one might just bust your VPN. You also forfeit expedient location changes using it and worse, now have to trust them to not have bugs that might subtly break the VPN while they build out their firewall/DNS features. This is a big ask when the VPN compatibility is secondary to their product. If that wasn't bad enough, all of this is off the table if you rely on obfuscation/anti-censorship measures in the full VPN app.
Something similar to the 'Block connections without VPN' or 'local sharing' toggles that exist in the OS and VPN apps makes these problems go away. Things become even more intuitive and seamless with the upcoming Private Space feature in Android where you no longer have to deal with switching profiles.
Another high privacy/anti-censorship use case enabled by this that is increasingly relevant these days is nesting VPNs, explained here. You could have your more privacy-respecting but censored VPN running in the Owner profile, sharing its connection, and the less privacy-respecting but more available VPN in a sub profile or the Private Space without exposing your IP to the nested VPN. This is iCloud Private Relay without being limited to certain countries or the Safari browser.