I had a strange feeling about people advertising the dead and closed source Gnutella client BearShare, but I only found one of the reasons for that gut feeling today.
Assumptions I use: We want Gnutella to continue to evolve and grow better.
To have Gnutella evolve, the developers of actively developed clients need feedback (and be it only encouragement).
If people now use a dead client, which won't evolve anymore, they don't provide essential feedback to actively developed clients, and it might even happen, that some developers waste time on trying to hack the dead client to make something work (again), instead of contributing to an active open client.
So every user who uses a dead closed client instead of an active open (and free licensed) client hinders the evolution of Gnutella.
That's not the fault of the user, and it's not per se damaging to the current state of the network (as long as the user shares, he contributes to the availaable files), but on the long term it hinders Gnutella from becoming better.
And that in mind, promoting a closed dead client directly damages Gnutella.
I know I'm human and as such prone to errors, so if you see anything I overlooked, please tell me about it.