Does the 73 2.0 L need the de-accelerator valve to operate correctly? What problems, if any, may occur without it?

No. It allows extra air into the intake on deceleration as an emissions control function. The 70-71 cars didn't even have one.

I've never ran with one and never had any problems, or noticeable differences in running. It eliminates one more thing that can go wrong.

Some may argue that it is easier on the MPS diaphragm with one.

I heard/read that too. Interesting that an L-jet also has a decel (and no diaphragm).

No. I ran my 1.7L D-Jet without for years with no apparent ill effects.

It does change the throttle response, the engine decelerates faster. RPMs drop quicker and there's less "coasting".

That was in fact why i removed it as i wanted a more direct throttle response when letting off the gas.

That is because they put it on as an emissions device on both the 2.0 and 1.8. It is quite common device on many car, alot of cars. On deceleration the high engine vacuum causes a very rich fuel mixture. The decel valve opens when the vacuum gets real high (closed throttle while decelerating) helping to reduce the mixture and emissions.
That it may help the life span of the MPS is just a fringe benefit.

As Andy points out the lack of engine braking effect is noticeable and was always my biggest irritation with it.
Jim

I pulled mine because it was leaking. No performance change however a small amount of a muted popping is noticeable when decelerating a cold engine. After 2-3 miles it goes away.