Good point and there are quite a few cases where fan spacing doesn't matter too (TJ-07, various Lian Li). But TC is just one manufacturer amongst quite a few others (Black Ice, XSPC. Feser, Swiftech...) which all use 15mm. It stands to reason that case compatibility and accessories like grills will never be as widespread for a less popular fan spacing. It makes sense to jump on the 15mm band wagon.
The bleed screw on my PA120.2 has had a slow leak on occasions but I've spotted it easily while leak testing and fixed it. It's a shame this will be going as I've found it quite useful for draining when changing coolant without having to disconnect/remove the rad. But if it means they are easier to make and cheaper as a result, I won't be complaining! I think it's good news.