Well, Angsty, you are not alone in being suspicious of the wisdom of polypharmacy, as it often gets practiced. You are in some very good company. The criticism that I read of most is not about concern of interaction issues (though that's a concern, especially when multiple drugs have similar bad side effects that become additive) but there is concern as to whether layering on multiple drugs actually does any good. Here's a statement by a psychiatrist from Columbia University:

"What we practice now involves the use of multiple antipsychotics in combination or an antidepressant, a mood stabilizer, benzodiazepines, or anxiolytics added to antipsychotics. Clinicians do this because the antipsychotic by itself isn't working sufficiently well, and they are trying to improve the level of response. But little evidence supports the use of these additional medications, and virtually no evidence supports the benefits of combining antipsychotic drugs."

From: http://www.medscape.com/viewarticle/756977