You are making the assumption--untrue categorically--that Chicago's collecting pattern is entirely summed up by the acquisition of a major work by an artist at the height of his popularity. I draw your attention to the fact, for example, that American museums and public spaces routinely incorporated Modern and Impressionist work into their collections at a time when the UK and by extension Canada barely knew what such things were. This is why the Art Institute of Chicago has more significant Impressionists than the whole of London's public art collections put together. Chicago itself has a strong tradition of nurturing local talent.
But even it were this not the case...let's put it this way. I suppose we can pretend their Picasso means less than it would had it been bought when no one knew who he was. But that smacks of the worst kind of adolescent "I know cool bands--and you don't" snobbery to me. At the end of the day, they still have it and it stands or falls on its own merits, not the method of its acquisition.
BTW, here's an article on how we treated those early works by di Suvero. Not very well, it would seem.
http://www.livewithculture.ca/art/flower-power-blooms-again/