From the teams perspective Cal I'd think testing is more important to cover up than at races.
I see their argument as running something like this:- All sorts of bits and bobs are tried out on cars in testing, some work some don't.
However by allowing everyone total access to see everything means other teams - particularly the bigger better funded ones - can more easily corelate lap times with certain added goodies finding out how effective they are almost as quickly as the team themselves.
Sometimes the whole point of running some extra little fins, under floor tricks, or even big funny shaped wings, is to try to lure the opposition in spending time & resource investigating a red herring.
They may not be able to slow down a faster car, but they just might succeed in slowing it's development further.