It clouds your judgement whether you know it or not. I am not saying it is the worst thing in the world but it is advantageous for the company buying the lunch, otherwise they would not be paying for it. I do not think that taking a lunch here or there is a huge problem, but I would disagree that it has no effect.
An example from my work: We were given a lecture from a pharmaceutical company promoting a medication for restless leg syndrome. The presentation fell for all of the classic problems of drug company funded studies and the way you can design a study to get positive results(in fact, I use that lecture to teach medical students about evidence based medicine and pharmaceutical company bias). Basically, the medication did not perform better than first line therapy but after hearing the lecture, I now know about the medication. I will clearly prescribe the older, cheaper drugs with similar side effect profiles but if I had a resistant patient who was suffering I may be tempted to try this medication even though the evidence did not show it to be more effective. Despite my best efforts, I now know this drug exists and may use it and this is only because of lunch talk. The better way to learn about new drugs is trials in independent, rigorous peer-reviewed journals, or by guidelines from your specialty.
The nominal "I am just gonna leave this here," or "Oh, I forgot this." Officially, my company does what it "needs" to do. I get that. Granted, this happens more at the off-site building where I work, rather than the main building. Less eyes that are all RULEZ!!!!!!!
