Yes, you can certainly do it that way, and it is probably how most people calculate RMV. However, it is very useful to know RMV under different situations: Being calm and still for deco gas planning, normal movement for 95% of dives, and under work/duress for high energy environments or bailout gas planning.
You might go through all three types on a single dive if you have some current to swim against at some point, and then do a nice 5 min safety stop at the end. The differences will get lost using average depth and total gas used. That may not matter at all (or might even be better) for no deco dives, but may be important for gas planning. As a newer diver, it's probably useful into know how much of a difference there is between working and resting.