Why not useful? In my experience setups involving IP ACLs for TCP services are pretty common in the wild. As well as risk-assesments talking about the relative rarity of on-path attackers, predicated on TCP security against off-path attackers.
Risk assessments making bad assumptions is not a new thing.
People are always advocating egress filtering because "if only everybody would do it ...", but it's a classic tragedy of the commons. Egress filtering doesn't meaningfully help the network doing it and it may cause ugly problems with asymmetric routes and the like, so the number of networks that don't do it is large enough to be meaningful. And if you get close enough to the core of the internet it's basically impossible anyway because there is no practical way to keep track of which address ranges a particular interface should legitimately be sending traffic from when the list encompasses half the address ranges on the internet and can change at any time.
The upshot being it's not at all difficult for an attacker to get hold of a connection that doesn't do egress filtering, and that isn't ever going to change.
