I've been using gravity and a nice neat fit with my router plates, for years. I was sceptical when I first saw the technique, but it works just fine. I would say, ensuring your router table is made of something that wont expand/contract; you can make the plate a snug fit - it wont go anywhere.
Having said all that, 3 years ago I bought a Triton Router that let's me change bits above the table anyway! (See my signature pic on the left)
Having said all that, 3 years ago I bought a Triton Router that let's me change bits above the table anyway! (See my signature pic on the left)