There should be more than one implementation of the bitcoin protocol.
Also, there might be legitimate use cases for "enterprise" style full nodes etc.
I could think of a thousand reasons why XT could be useful, but the most important ones will likely be the ones I can't imagine right now.
There are a lot of 'implementations' of the Bitcoin protocol. Just look at the debug.log from Bitcoin Core and you will see lots of different 'subver' strings that are, effectively, user-agent strings for different clients, many of which are not Bitcoin Core.