Hi
I'm no expert, but IMO even though there's no movement in the diff, the engine and gearbox move a bit on their mountings so I think the splined joint is still needed. If it wasn't there you would be putting a lot of end load into the gearbox output shaft. The same applies if you were even a couple of mm out in specifying the length of the shaft.
I'm pretty sure that Westfields have sliding joints on their shafts -
http://www.westfieldbuild.net/propshaft.php.
Matt