I just posted a message about this yesterday to the "Time Travel...Yeah I know! maybe some parallel universe talk too!" thread, which was moved to this category.
It needs to be recognized, that time is a semantical concept we use to describe motion. For motion to exist, time dimension doesn't need to, and time is a derivation of observation of motion. (=Time is never observed in a metaphysical sense)
It would be very problematic philosophically, to think that time actually metaphysically exists as an actual dimension, like we visualize it in spacetime graphs.
But just for the sake of argument, let's say time dimension does exist fundamentally, in the way implied by Minkowski (world is a 4D-spacetime);
Nothing moves in 4D spacetime. But if nothing moves, what causes our (conscious) experience of time flowing forwards? Surely it must be something in physical existence, some fundamental, which is not present in this interpretation of relativity.
But if instead it is motion that exists fundamentally, there is real, metaphysical motion in our brain processes, and this would give us real sense of what we call "flow of time".
Another idea is to include a concept of fundamental 3D-slices to each matter particle, that metaphysically moves through the 4D-spacetime making motion real to us. But for anything to move through the time dimension, there would need to be another notion of time to describe the motion of this metaphysical 3D-slice through time. This leads to obvious infinite regress and is just far too naive worldview to be considered true.
It has been said that time is an illusion and any slicing of 4D-spacetime is completely arbitrary, but it should be plain to see to anyone dabbling in philosophy, that our conscious experience is the only thing we know for sure to exist, and our experience of time flowing forwards cannot be denied just because on spacetime diagrams nothing moves. Indeed, it would need a special mind to deny one's own experience to be real, just to make the mathematical equation of reality look simpler on paper.
Another related question is, does time have to be a dimension for relativity to work? Well, no. At first it might seem that relativity of simultaneity could not exist if time is not a dimension, but actually the idea about relativity of simultaneity is also just one interpretation of the math. It is probably impossible to make any clear ontological interpretation of relativity anyway.
For example, can we suppose that planes of simultaneity really describe the state of the world around us, and therefore clocks around us actually move at different paces or even backwards just because we change directions? I don't know of a single interpretation that would not be problematic one way or another, which is not to say the math couldn't be right. (we should expect reality to be very very elusive to our semantical minds anyway)
And btw, another related question is, what is "space"? Does space exist, metaphysically? What do we even mean with space? When there is no matter, is there still this invisible backdrop that exists, or does there only exist matter? Well, metaphysically, the concept of space doesn't make much sense, only the concept of "distance" does (which, like "time", is a matter of taking one physical thing, and simply comparing it to another physical thing. Even if there are no metaphysical distances, there still are comparisons between things and the differences are real metaphysically)
Think about an atom, for example. Where does the atom end and space begin? We know there is actually no "boundary" to an atom in any real sense of the word. If we scaled our size down, we would not find a "wall" of an atom. The location where one atom collides another atom is just the distance where their electromagnetic forces stabilize and keep them from collapsing together. This is just a physical function of the stable system we call an atom, so can we really say atom is a "particle" in "space"? No, we can't. It is a particle in space only in the specific mental model that we use to classify reality, but metaphysically atom is extended and if it has any size, the size is its whole area of influence, which is potentially the whole size of the universe. We are swimming inside extended atoms all the time.
All the observations of quantum realm are strongly indicative that the above paradigm would be in fact more useful way to understand the reality than standard model where particles exist like little billiard balls. (No, matter is not made of matter, btw :)
Oh well, my $0.02
