I think knowing how much water ballast you're carrying would be a good thing. Probably more for convenience than anything "essential" for the subs operation. It would give you a little leeway in buoyancy while at depth which could be very useful for several reasons. Maybe you wanted to settle on the bottom, or pick something up, or jettison a buoy, or drop some markers, or any one of a host of things that could change your weight while diving. Knowing how much reserve capacity in buoyancy was available could be very useful.
Then again, at the beginning of a dive, you can estimate and pre-load your VBT with as much additional weight as you will need to drop once at depth, by knowing how many gallons are in there, and how much room is left.
With a front and back VBT, you can change dive angle slightly by pumping water from one to the other. It would be "convenient" to know how much you transferred, and what angle that produced, so that the maneuver could be repeated with accuracy, say 18 gallons for a 10 degree down angle.
If a means to measure the volume was reliable, reasonably accurate, simple to maintain, and absolutely leak proof, that would be worth putting in.
I like the idea of the see-thru propane tanks, but that will limit your depth to 250 psi. or about 500 feet. Now that's what they're rated at for internal pressure, and I'm not sure what the safety factor is on those tanks, but they come with plumbing fittings molded in, and you can see at a glance how much water is in them. (That's reliable,accurate, no maintenance, and don't leak.) Still, It's not "essential" to even have any VBT