No question, wearing a habit, cassock, or clerics for that matter, is a clear witness to the world of a life lived for Christ.
Watching EWTN morning Mass one day, one of the Friars was preaching, in part, about wearing the habit. He talked about visiting a convent of the Poor Clares of Perpetual Adoration in the Southwestern U.S. with several other Franciscan Missionaries of the Eternal Word (MFVA). He told how they were in a restaurant eating, and as they would, they forgot about the fact that they were dressed a bit differently than the normal customers. He went on to say that at some point their young waitress came out and said, "everyone wants to know, what are you guys?" The friar answered fairly succinctly, satisfying her curiosity as she went back to the kitchen to relay the answer. A little while later she came back and asked more questions like, what is a "Franciscan"? Several rounds of questioning later, she said she didn't "know God". At that moment, one of the lay brothers answered immediately "You may not know Him, but He knows you, and He loves you!" The Friar preaching the homily jokingly said in his homily, "man, why didn't I think of saying that!"
The Friars may never know what will become of that young woman, or the other employees that were asking questions of them, but one thing is for sure - No habits - probably no conversation about the Love of God.
Second story. A few weeks ago a young man came into my office to talk about vocations. What was driving the beginning stages of his discernment? He had been visiting the Grand Canyon, and ran into, all of people, Franciscan Friars of the Renewal (they have a friary in New Mexico). He spoke to them, and it got the wheels turning. Weeks later he's in my office asking all the right questions. Again, no habits, and this story doesn't happen.
Ask any religious that wears a habit and they probably have a dozen stories just like this. I'll never believe the relativist idea that "I don't need to wear a habit, my habit is living my life according the gospel." Maybe, but you'll never be a visible witness, to someone driving by, of a truly counter cultural life. "They're uncomfortable to wear" - I'm glad Christ didn't say the same about "wearing" His humanity when it became "uncomfortable", and give it up for "casual wear".