While it doesn't necessarily determine one's skill, it at least gives you somewhat of an idea as to whether or not they're worth a damn.
I'd rather fight someone who's 400/100 than someone who's 4294/3829.
W/L means less than nothing so long as you can just use TrueSkill instead.
In Ranked I have waaaaay more wins than losses.
But in Player matches I have much more losses than wins.
The reason is Im more willing to rematch people who beat me than most people I play, given they usually leave after they lose a couple times. So in Player matches where I can rematch, I naturally lose more.
Even though I have a better record in Ranked, my TrueSkill is higher in Player matches because it takes into account most my loses are repeats against very strong players.
Obviously, you should be using TrueSkill, as its a better measure and cant be as easily manipulated as W/L ratio.