If you have a known power setting and a known pitch setting then your speed will be (within some tolerance) a fairly stable figure.
However, as you say, knowing pitch and speed doesn't tell you very much apart from the fact you are still moving forwards as the climb or descent part of the 'performance' will depend on the power setting.
That is why it is quite possible to fly an aeroplane with no airspeed indication if you know the rough power and attitude settings for various speeds and configurations.
Similarily you could set a pitch attitude and then adjust power to achieve a speed and either climb / descent or level flight if you had no power indications,
However as MJ rightly says, the basic phrase power + attitude = performance works for most circumstances