I'm of two minds on this. On one hand, articles like this seem to indicate that Sony has become incredibly developer-friendly... but on the other hand, the recent Xperia phones wipe a DRM partition if you unlock the bootloader, irretrievably destroying keys that allow a bunch of things not work, such as some software that makes the camera actually good.
So we're stuck either not unlocking and rooting at all, or waiting for an (unlikely?) root exploit to be found that allows a custom recovery to be installed. And even then, running a custom kernel still requires unlocking the bootloader, so you can't run AOSP or CM, or any of the other mods. (Fortunately, with a root exploit, you can back up the DRM partition and restore it later, but still, it's not ideal.)
It's not just recent phones. This has been in use since at least the Xperia S, though back then you hardly lost anything by wiping it lol. I've been wondering for a while now if Sony could use eFuses (such as the qcom qFuse, seen very recently on the Nexus 6P) to store data, but I've yet to see any change. Perhaps with the Z6 line that launches with 6.0?
You do not really need root to run a recovery, as it's the bootloader's (and until a few days ago, the boot image's) responsibility to boot it up, meaning you need to have your bootloader unlocked. (Unless we're talking of /system recoveries, but those are hacky workarounds at best.)
A few years back there was a CM10 build that could run off the stock boot images, so people who had root but also had locked bootloaders could very easily run CM.
So we're stuck either not unlocking and rooting at all, or waiting for an (unlikely?) root exploit to be found that allows a custom recovery to be installed. And even then, running a custom kernel still requires unlocking the bootloader, so you can't run AOSP or CM, or any of the other mods. (Fortunately, with a root exploit, you can back up the DRM partition and restore it later, but still, it's not ideal.)