In my experience, this ad placement strategy is usually suggested by the ad rep who jumps on the opportunity as soon as he or she learns that the editorial feature is scheduled to run. It is generally pitched as a means to increase exposure, but in reality it has the opposite effect. Here are three main reasons:
1) Editorial content is generally regarded by readers to be an unbiased, unpaid for, third party endorsement. Placing and ad opposite such a feature gives readers the impression that the editorial and ad are merely one big advertorial. This nullifies the credibilty that the editorial would have on its own and undermines what the overall PR program is striving to achieve in the first place.
2) In my view, it is better to expose the reader multiple times to the client's branding message whether it be in the form of editorial or advertising, rather than to crush them under one big editorial/advertising footprint. Separating the ad from the editorial content accomplishes just that.
3) When advertising budgets are tight (which seems to be most of the time), PR can help supplement a limited placement schedule. Why not move the ad placement to a month where there is no editorial feature running (or vice versa) to stretch the media budget and ensure more exposure thoughout the year?
So the next time an ad rep suggests placing an ad opposite a feature article about your client or your company, remember the words of Betty Ford and "Just say no."