When to consider going to the dark side of 650b?

[ridethecliche]

I know folks have talked about this a bunch but I've been thinking about it lately so wondered what the calculus is these days.

It seems that 650B was a stop gap when frame tire clearance was pretty limited to 35-38mm. Is there still much of a reason to go this route today when new frames clear 700x50+?
Outside of frames built with 650B in mind.

In my use case, I have a giant tcx advanced sx. Based on my measurements I have 55mm between the stays for 700c and 64mm between the stays where a 650B would go. Id say clearance is for 45mm and 54mm tires respectively, the latter of which is a 2.1.

What use scenarios have you found that dropping to a 650B is worth it? Honestly I'd have to test fit to see how things play out but there's more room back there than I anticipated for 650B