Microsoft Audience Network gets switched on because the CPC looks great in week one. The clicks are cheaper, but the placements are mostly display-style inventory and the intent is usually weaker than on the search results page. Say a B2B software vendor with a $300 CPA turns it on across the whole account. Clicks go up and cost per lead may even drop on paper, but the leads are often junk and the search terms report gets messy fast.
My view is to keep it off the main Search campaigns. If you want to test it, give it its own campaign with a tight budget and a CPA cap, and judge it on qualified conversions, not click volume. If the numbers hold after a few weeks, scale it. If not, it's just cheap traffic.