James,
You are correct, the intention is not server-side conversion operations but rather to facilitate c# code LINQ type statements. I originally tried to do the (x is AThing) but of course MongoDB wouldn’t understand that and I’m not sure the discriminator is meant to work that way, my understanding was that the discriminator was more related to the de/serialization process.
I will file the bug accordingly. Thank you for responding to this with meaningful replies!
Ticket Made: https://jira.mongodb.org/browse/CSHARP-4522
If I did not provide information just let me know