I've used them but in rough-work areas like food processing plants, engineering workshops etc. where there's more than spilled beverages to worry about. Even then we replaced keyboards every six months or so due to contact failures from crud getting past the barrier.
The silicone covers won't stop spills getting into the rest of the laptop, just the keyboard itself. I'm a desktop user in the main and I'd typically not be using a laptop with a drink to hand. Saying that my coffee mug is chunky with a heavy base (I got it decades ago as a freebie at a WordPerfect for Windows roadshow) rather than being stylishly tall and slim and unstable.
I have a bad habit of eating while doing stuff on my laptop. It's the first time I've spilled anything on it, so hopefully this will not be a recurring problem, but it will help to keep crumbs out of the keys etc. so it's probably worth doing anyway.