oh I prefer this to anything hardcoded any day but it’s not what is friendly for me (as a tech artist), I just mean artist-friendly in a level designer sort of way.
if the setup requires just copy-pasting a few nodes then there’s no reason to not have it under the hood, but if those nodes need tweaking then it’s likely that a level designer won’t understand what needs to be done because a level designer doesn’t necessarily know how to create complex materials or what even is WPO (and their materials might consist of an and a normalmap hooked and that’s it). in the end it’s the level designer that will use the feature, and not every team has tech artist to set things up for them.
I’m just going here by what I see as Epic’s usual behavior in terms of tools usability. usually they try to go for something that “just works” or comes close to it (as with mesh decals’ WPO) and ignore when the flawed use cases are uncovered (again as with mesh decals), in other cases where finetuning is more necessary they usually try to go further with editor-exposed variables.
in this particular case I think they’d need to expose a variable in the “spline decal actor” that is passed into the shader, or maybe a variable in the material properties, but again I really don’t expect Epic to make a specific WPO material node setup to be -required- for a feature like spline decals.
of course this is all just speculation from me, but think I’ve seen enough of how Epic thinks around their tools to have an educated guess 
if the vertical error can be figured out in 95+% of the cases, I would think they would just hardcode it in the shader. this is the “just works” case I mention above