I suppose running Bitcoin Core without port 8333 is useless for the network, is it?
I wouldn't say that it is useless, at least you're doing more than most. Per my understanding, port 8333 is meant for inbound connections which is the channel that allows other nodes to connect to your own node but in the event that you are not able to open the port, you still provide the network with some benefits, per Greg Maxwell reply here[1]:
An outbound connection is functionally equivalent to an inbound connection except in the respect that you initiated it. An outbound only node provides the same resources to the network except that it doesn't provide incoming connection slots.
(...)Nodes without open inbound also provide important DOS and privacy protection to the network: It is much more difficult for an attacker to get connected to nodes that don't accept random connections, since the attacker must wait for the victim to come to them. So the presence of outbound-only peers has the advantage of strengthening the network against some kinds of attack. (That said, the network has more than enough out-only peers to achieve those benefits already...)
Would it be optimal for the node to have its inbound connection port open? Yes, but if you are indeed unable to do it, you are still, in a way, contributing to the network.
[1]
https://bitcoin.stackexchange.com/questions/88127/are-nodes-that-dont-accept-inbound-connections-necessarily-leechers/88147