I would say that yes knowing how to manually calculate W & B is a good idea. Some of the newer tools that are available for flight planning are great, but what happens when the internet is down, or you have no access to them. I personally like the AOPA flight planner but it should not be a substitute, only a supplement. Same with weight and balance. A few years ago I had a co-worker at an FBO (non-pilot) want to take a ride with me. When I got there he boasted that he had already done the weight and balance and an instructor helped him. I took one look at it and said "this is wrong , I can tell just by looking at it". Then did it manually. He used one of those pre-made excell programs. The arm calculations/numbers were all wrong. It's best to stick to the POH and be cautious of outside resources. They are useful, but the POH is the actual resource. Imagine if you got ramp checked by the FAA and had nothing but supplemental resources for your calculations and none of them came from information derived from the actual POH. On a another note I personally feel that while flight planning and using the POH for information it's still, ( Measure with a micrometer and cut with a chainsaw ).

The instructor may have felt that you needed a little more ground work as far as wanting to do more ground. It's best to catch up with the times, or get left behind.