A lot of things may well be - but there is a core of reality. 'Neti neti' (not this not this) implies that the divine cannot be defined, yet there are millions of pages of defintion coming from many different traditions and sources. I think that sometimes, all that can actually be a big barrier to any actual realization. In extreme cases, people will fight and die for what amounts to no more than a set of mental definitions. The fact that these definitons are shared by millions doesn't mean they are correct or even useful. They may give what one could call mental security - in the face of the deep mysteries of the universe a person may decide that this or that partial explanation is it - but in fact it is often more like a superimposition on reality.
So this brings about another question, is "Neti, Neti" defining divinity as the undefinable? haha, if you turn the logic around , you basically stop talking about spirituality all together. I don't think the problem is a "definiton of divinity" or even a "definition of reality", the problem of superimposition arises when such definitions about the divinity claim that they include all aspects of the divine. If the definition claims that it only partly understands the divine being, then maybe it is harmless and even helpful to have it. It is like a mathematical limit in calculus, obviously, the series {1/2+1/4+1/8...1/nth} may come close to the number 1, but it may never be 1, but it is so close that it suggests the real number is closer to the number 1. Similarly, a definition may not give all aspects of the divine, but may come close to grasping the entire truth. But I think "neti neti" suggests that it can never come close to defining reality, such a definition destroys the chance of even coming close to a meaningful realization of divinity.
The thing is though that the divine is undefinable. Inconcievable - infinite - so how can finite mind define it? Where definitions are concerned, I agree they may be useful provided they are not seen as absolute, and are open to be expanded, discarded or changed as realization proceeds. But many systems of definition have become enshrined as absolute dogma, and are held by some to cover all aspects of reality. I'd say that in the end, all definitions have to be transcended in an experience that transcends ordinary mind. In one place, the 13th c domincan master, Meister Eckhart (IMO one of the greatest of western mystics) says: 'we must not be satisfied with the god we have thought of'. Because that is precisely a thought - a mental image, and not God itself. It's the same if I think of a tree - it doesn't compare with seeing the tree, touching it. The thought of the tree, the image of it in my mind's eye even, is not the experience of the tree. I think ultimately, 'neti neti' is more about seeing the limitations of mind and of mental definitions. But of course, it's only one way of looking at it all - I think there are many ways and paths that lead to the truth. Neti neti is saying, in a sense, 'God is nothing you can think of'. You could say 'God is everything you can think of' - both are true in a relative way. But both are simply more mental formulation too.......
Just to add - it's probably like the Zen thing about confusing the moon with the finger that points at the moon. It's ok to have ideas about the divine, but one mustn't take them for the divine itself. One of the good things about Indian yoga traditions, and which first led me to become interested, is the whole notion that yoga isn't saying 'read this book/ accept these beliefs and you'll know the truth', but 'try this practice and see what experience it leads to'. That's the kind of approach I think is useful in terms of spirituality.