Using average sizes for objects/dimensions can make the hitting a target more accurate without knowing the exact size of the target but yes it would be less accurate than knowing the exact size. That said if you don't know the range at all you don't know what to put on the scope so you can't take the shot anyways. A LRF is a much preferable way to get range over using the reticle but the skill of using a reticle works.

All that ranging stuff said, it's actually low on the list of the actual use of a reticle. Using it for hold overs and unders, wind holds and movers holds are used much more frequently than ranging especially with almost everyone having LRFs now. In that, having a FFP scope makes doing all those things easier as you don;t have to worry about what power you are on or having to be on a specific spot on the power knob and doing match to figure what the reticle is subtending on that power. With a FFP scope you just dial the power where it needs to be for you to have the proper FOV and clarity to take the shot and then use the reticle. Simplicity makes the job of hitting the target easier.