Having read a thirty page monstrosity on this topic elsewhere, I'm curious to see a slightly more reasoned discussion of it.
In the United States lawyers are recommended by the American Bar Association ethical rules to contribute at least fifty hours of pro bono service per year. Whether or not individual attorneys should feel obligated to follow this recommendation, why should they (or why should they not) be recommended to do this in the first place?
On a related note, law firms are ranked in part by their pro bono participation and this information is passed on to law students. Is a law firm with low pro bono participation likely to attract the best law students? What about one with high pro bono participation? Does it matter either way?