I've always been a HUGE note taker. I also 100% agree that there are many players who will have a similar playing style... with similar tendancies (years ago I would assign color tags to these styles I had listed (with over 10 different player types at one point)).
And for sure you will constantly run into the same players even in tournaments with massive fields. The bigger your note base becomes, the more you will see them (obviously). Back on Fulltilt I had a massive amount of notes & would keep them stored on a memory stick which I would update regularly. There would be occasions where I would be in a tournament with 100 players remaining & I would have notes from previous occasions on over half of them. In Sng's I would regularly have previous notes on the entire table (in the 45mans).
I played a ton of private games back then, across a couple dozen different sites (or more). It was the same pool of 2 or 300 players from countries all over the world (with some different ones in each group/forum/club, etc.). Extensive notes in those games helped me to get some great results! (I used to do okay in the CC games too)