I also keep the Vallaslin. For basically what the in-game line says: "Whatever they meant in the past, they're important to me now."
(or... something close to that)
"Whatever they meant in the past, the Dalish have reclaimed them. They mark me as one of them." 
If Solas presses, I love choosing "No, they're important to me." Lavellans says out loud, "Even if what you say is true, I don't think I can just let you erase them."
When it's crunch time and you have to make a choice, I love what they both say when she chooses to keep them.
Lavellan: I know you told me because you wanted to help, but [the vallaslin] is part of who I am. If you could only see past...
Solas: Please stop. *smiles* You are perfect exactly as you are.
AWWWE! 
In all seriousness, yeah. It's been thousands of years since Arlathan, and it's implied the Dalish have picked the them up only in the last 700 years. It's inevitable that they would have different meanings now from ancient times. Yeah, their ancient function was slave-markings to honor their master's gods, but now they're willingly taken on by free people to honor their gods. More than that, vallaslin have become ritualistic rites of passage that mark elves as members of the Dalish, and symbolize their dedication to their heritage, tradition, culture, and people. My Lavellan always felt her vallaslin symbolized these things, and felt that discarding them because they used to have a shady meaning would be symbolically discarding everything good about her heritage, culture and people because it used to be bad. And all people's histories and cultures have flaws, so why throw the baby out with the bathwater?