Jeff argues that displaying both values would increase server load by unnecessary queries.
However, everyone, if they want to, can use rep graph, a legitimate SE tool, to get some result (although, not very accurate, as @Grace noticed).
By wasting more time and submitting further queries on SE servers, more accurate metrics can be obtained.
There are also other legitimate tools serving same purpose.
One more consideration. Rep graph can even reveal who has downvoted your Q or A. It does not apply users with 1000+ rep who are familiar with SE culture. Instead, for the new users who are most sensitive about downvotes, it can open a way for "revenge serial downvoting".
So I agree with OP: this limitation, in a current implementation, seems to be redundant. It does not protect SE servers from extra queries in any way.
If it is required due to whatever considerations, the SE developers should think on how to block any workarounds.
/reputation
, or by checking your envelope report. They're typically much easier to use than the graph since they differentiate things like bounties and capped votes.