Well, what _should_ it return?
What it returns right now is one of a list of hardcoded things based on XP_* ifdefs none of which would apply to Gonk, then falls back on the mOscpu that the HTTP handler provides. But Gonk happens to ifdef all that code out with MOZ_UA_OS_AGNOSTIC for some reason, so it gets an empty string...
What exactly is the intent of MOZ_UA_OS_AGNOSTIC, and how should it impact navigator.platform?
I'm pretty sure that we agreed that '' was the right value to return, given our position on not putting hardware info in the UA string either. See bug 777710, comments 10 to 16.
(Does this bug intentionally not have a comment 0?)
Based on Gerv's comment 2, I'm going to resolve this bug as invalid. An empty string is the intended value for navigator.platform on B2G.