Here's how I know it, although I could be mistaken, because I'm not really an audiophile. I do have a "stereo", but it doesn't have remote volume control, and my brother just recently got a stereo TV, and it does have remote volume, so I usually don't even turn on the stereo. I just use the stereo TV speakers.
Apparently Dolby Digital has something like a two second lag time, which makes it almost useless as a videogame technology.
Sony recognized that, so they "disabled" access to the Dolby Digital aspects of the PS2 while it's in "game mode". Even so, Sony fanboys used to make fun of the fact that the GameCube wasn't designed for DD, and laughed at Factor 5's claim that they were making Dolby Pro Logic 2 as a "surround sound alternative" for the GameCube.
Some PS2 developers didn't believe that DD was
totally useless to games though, and Microsoft was trying to push DD in games as an advantage, so Sony released some codes to developers that allowed them to temporarily go around the PS2's OS and "re-enable" DD in videogame mode.
They ran the background music as Dolby Digital, and ran the sound effects through Dolby Pro Logic 1.
Factor 5 finished Rouge Leader, and gave their new custom-modified version of DPL2 back to Dolby. And Dolby gave it to interested developers.
So now there are PS2 games appearing with DPL2 instead of DPL1, and sometimes some Dolby Digital mixed in.
I think there were also some other games claiming to be able to run
everything in Dolby Digital. But for the most part, if games have DD or DPL2 in any capacity, they just list themselves as "having" that kind of sound, and don't say anything about how much of it they're really using.
So I don't know how many games actually used DD, but I think the number was small, and I think it was primarily something odd like "most EA sports games". I think DPL2 is being supported about as often on the PS2 as it is on the GameCube.
I'd say just skip the optical cable until you see "Dolby Digital" in the starting credits of enough games that you like to make you change your mind.
Unless you find the cable cheap somewhere (I'm a big fan of that).