Wait, you get RWX pages? Is that new-ish? Last I was paying attention to this (around iOS 7 and 8), the kernel support for Safari's JIT gave it a single RW page at a random address, that you could flip to X (not even RW) when done. What's the use of concurrently modifying executable pages?
Maybe I'm confusing this with some other system?
Its usefulness can be debated, since ROP and other techniques are powerful enough for a motivated attacker to go a long way without actually executing code, but it's not fundamentally invalid as an exploit mitigation technique. In fact, PaX implements something similar:
https://pax.grsecurity.net/docs/mprotect.txt
Of course, Apple evidently thinks the speed benefits of JIT are important enough for Safari to be worth letting the browser process not just remap RW to RX but actually map RWX pages - despite the browser process probably being the biggest target of all for exploits. I personally think iOS should allow apps to opt in to doing the same, but that's just my opinion.