Until neovim actually does something cool and new that you want to use. Ex mode isn't something I need often, but it's fantastic on a sufficiently slow connection (of which there are fewer these days) or a sufficiently broken terminal (slightly more common), and when you need it you need it to work.
When neovim does something new and cool that I want to use, then ex-mode-in-vim will obviously not support that new and cool something. If neovim continues to provide an ex mode, then it will be available to me.
Obviously, whether this makes sense depends on what the something is - if it's some GUI flourish, then it's irrelevant either way, but a GUI flourish is not going to be why I move to neovim in the first place.