If the user meets all the criteria and is approved, then there is no point in keeping him on the bench. It is necessary to give the opportunity to begin performing duties as soon as they are ready.
The requirement/criteria is very simple, as set by the admin, 1) be a reputable member and 2) collect and post 10 posts you think are of good quality and haven't received enough merits.
That being said, Theymos cannot immediately make everyone who applies and meets the criteria merit sources. I think he keeps some in the pipeline, and whenever he thinks there is a reduction in merit distribution in the forum, he can pick from pending applications and make new merit sources.