Nint has always been a gaming powerhouse, their ranking affects only 3rd party support, and even then being 1st doesn't help much. Also and most importantly, Nint manages to stay profitable whatever the situation of their console.... So for them ranking is unimportant.
MS is a money machine, profits are not important, thus sales are unimportant from this point of view. The only reason sales matter is because they are said to be in this to counter sony, thus making it important for MS to outdo Sony. So for them ranking above sony is important.
Sony thived on 3rd party support, they developped their 1st party but those quality games have only a moderate appeal to the masses (they are not the best selling on their own console, 3rd parties are). Also, Sony is inneficient when costs are concerned, they need high sales from software (and thus from hardware) to have their businness profitable. So for them ranking is important as it brings them the much needed revenues, being last is very bad.
Basically your reasoning fails when you say that loosing exclusive is unimportant for sony. The reason this matters is financial, they are the only company that needs a good ranking to stay affloat, and exclusives are what brings sales to a console. The quality of the Sony made exclusive is not enough to bring PS3 to the top (the 3 top sales are not sony made) and thus losing exclusive hurt them a lot.