The long-term consequences of constantly tapping into the dark side of the Force are left up to the roleplaying of the player and the GM. However, both should keep in mind that the dark side is born of fear, anger, and hatred, and these emotions should be present in the Player Character.
Having been unadulterated by forum perusal, as a GM, I took the above quote very differently than most people on these forums seem to have. I've explained it a couple of times in an older thread, but thought it bore repeating, not only for ideas, but for discussion.
I had a player who made a Thief/Exile and felt he could get away with dark side use of the Force regularly as long as he kept the Destiny Point spending in check, since they quickly learned that I love using Destiny Points, so they always have some Light to "waste".
So, my solution, based on the above quote, was to describe things differently with the very Destiny Point(s) he'd flip towards me. Other players would see things one way, but his paranoid, angry and hate-filled perceptions saw something other. The culmination of this caused him to forego the use of Force in general (at least, in dire circumstances, he'd still use it to move things when there was no time strain) and he ended up becoming a Slicer in lieu of a force-filled thief.
The Culmination scene (as accurately as my memory can recall it):
He is being hounded by a member of a bandit group, someone whose actions and description makes you just "love to hate" him, on one of the planets in the outer rim. The member has blamed the group, and this character in particular, for some events that have transpired, and threatened to call the Imperials (a bluff of course) to report one of their "Jedi" types.
The player decides to toss this guy as far as he can with use of "Move", not only for the annoyance he's been, but because it is part of the end-goal of cleaning up this area of banditry. He rolls his 2 die and gets 3 dark side force (and uses all 3, giving me 3 Destiny points). Tosses him over a building into the distance to his ultimate death.
I reply that (1 destiny point spent to alter his perception) he then sees a small figure dart from the shadows of that building, with a deadly weapon drawn, pointing it at the character, while laughing maniacally. Declaration of action...
To the rest of the group, who wanted to jump in immediately, I tell them, after the one player has declared his action, that they see a female child, the daughter of one of the villagers who has come to this group for help, come running around from the side of the building, giggling at something in her head. She has a pink plastic toy gun and points it at the group playfully.
I spent about 4 Destiny points to interfere with the other players interfering with the Dark Side. The child is slammed into/through the building by the 1st player, breaking half the bones in her young body. Although she does recover, she's permanently scarred, both mentally and physically.
To me, this is what is meant by the quote from the book. He realized that he was willingly giving himself over to the dark side and made the decision to not risk it again. It became more than just a mechanic of the game.
Admittedly, with the coming Morality feature of Force and Destiny, there is already a strong deterrent coming for use of Dark Side Force points, but once you're sporting 4+ Force, you don't really have a reason to use dark side anymore anyhows. However, my system, which I continue to use, albeit not as often, since my players are far more leery about drawing from the dark side, I think best quantifies use of the dark side when drawing on Force.
Not something you could really make a "mechanic" for, as it's all in the storytelling, but something that has proven to be highly effective for me.