The etymological fallacy does not state that knowing the etymology of a word will lead you to misunderstand its present meaning, it merely states that the original meaning is not necessarily systematically equated to the present meaning which, on the face of it, seems a fair statement.
The etymological fallacy is a genetic fallacy that holds, erroneously, that the historical meaning of a word or phrase is necessarily similar to its actual present-day meaning.
That is however a reductive use of the word etymology itself.
Etymology cannot be reduced to the original meaning of a word (if there ever was such a thing - words themselves having a life of their own). Instead it has to be envisioned as the whole path that leads from former meanings to present day meanings. It is not a static snapshot taken at an arbitrary point in the past, it is, as much as one can reconstruct it, the whole history of how words and cultures interact with each other to lead from stems to words and meanings. It is a dynamic thing leading to the present.
Let's take just one example: to tally. If you look it up online in the the free dictionary, you will discover a whole list of apparently unrelated meanings such as:
- To reckon or count.
- To record by making a mark.
- to score (a point or goal) in a game or contest.
- To be alike; correspond or agree.
The way these seemingly unrelated meanings are actually connected becomes glaringly obvious if you know the etymology of the word:
The base meaning of a tally is "a tick marked with notches to indicate amount owed or paid" (see French une entaille).
In the old times, people would keep counts of what they owed to each other by dividing a stick in two pieces and each time a new debt was contracted a new notch (a tally) was carved into the reunited pieces. Each party would keep its own piece and every now and then, accounts were settled with the two pieces matching. Because two different sticks can't be broken in the same way, there was no possible argument.
This is the kind of gem etymology brings to the knowledge of a given language.