You're Facebook. You want to provide an instant messenger you can iterate on quickly. You can put the work into making it XMPP-shaped and working with the community (whatever they means... Are you going to slip ship targets because Pidgin doesn't work as a client for your implementation?). Or you can just write and ship your own service and client at your own pace and not care about any other developer's needs or wants because you're trying to serve Facebook users, not developers.
It's true Pidgin 2's XMPP support sucks, there's no denying that. That said we can't easily fix that with the current API which is why Pidgin3/Purple3 is the only path forward.
That said, I'm pushing to have an experimental release of Pidgin 3 out by the end of the year. However it's only going to have IRC support right now because well there's only so many hours in the day especially when Open Source is your passion project and not your day job.
Unfortunately Pidgin was one of the most popular ways to use XMPP back in the day, and this had a knock-on effect on XMPP's reputation as an out-of-date technology. People still install Pidgin today and tell me, yep, XMPP hasn't changed since 2006. Which couldn't be further from the truth.