This is a common practice in a number of other fields (e.g. cognitive science and psycholinguistics), at least for conference submissions. In general I don't se

This is a common practice in a number of other fields (e.g. cognitive science and psycholinguistics), at least for conference submissions. In general I don't see a huge difference -- when a grad student has sufficient standing and expertise to be chosen by a reviewer, they generally know the domain very well and are often up to date with the research in a way that more senior reviewers often aren't. And because they have fewer demands on their time and feel a greater need to substantiate their critiques, they tend to write depthier reviews. And of course there's still the safeguard that the meta-reviewer can indicate to the authors in various ways that a review is garbage, or even throw it out / seek another reviewer.