I really can't understand why an inexperienced PhD student should be allowed to review a research article written by researchers of considerably more experience and expertise.
How do journals/editors select referees for submissions?
In many disciplines of science, asking Ph.D. students to review papers is considered to be fair by the community. I can only speak for computer science here, but the following arguments may apply in other fields as well.
When a Ph.D. student reviews her/his first papers, this is typically done for his/her advisor. Advisors often check the reviews before turning them in, and thus avoid highly unfair or imprecise evaluation of the work of others.
Reviewing work is one of the best ways to get started with writing your own work. You get to know first hand what can make a paper weak and what you want to read as a reviewer. This is very valuable to the student.
A single review on its own is seldom the reason for acceptance/rejection of a paper. If there is a strong disagreement between the reviewers, then this is normally discussed in the case of conferences (which are prime publication venues in CS), or the editor has a look and will gather further information.
Ph.D. students often simply take more time to review a paper. Thus, they can find flaws in papers that senior researchers overlook in quicker reviews.