My GTI has plastic retaining snaps molded into the carpet, and the WT liners have a matching hole that snaps in perfectly. It's a tight fit, so water doesn't get through.
The Yukon has no mat retention system, so the WeatherTechs have a hole in them and came with a little hook thing that twists into the carpet and hooks into the liner to secure it. But after kicking the liner a few times, the hook pulls out of the carpet and is now basically useless. The WT liners are stiff enough that they don't move around too much, but they do move a little. And there is a small hole, but it has a raised lip - not a big deal, IMO as I never let so much snow in there that it would ever really puddle up.
Bottom line, I think much depends on the vehicle. VW has an awesome system for securing the mats, and WT has used it to full advantage. GM, not so much (not at all, actually).