I think most of the blogs today are equip with earning ads. I don't know who started these kind of marketing strategy but there are already thousands or even millions (who can count?) of blogs out there using it.
Should we place these ads on our blog? I think should think first the balance the argument and consider some points before putting ads in your blogs.
How much would they pay you placing their ads?
Look who's advertising in your related niche?
Think of position in your market
Consider your Competing Ads and Affiliate Programs
What do you think about putting ads in your blog?