Professor Reynolds has a virtual copyright on this phrase and the philosophical approach implied by the phrase certainly ties in well with many of the other memes he promotes, but I wonder if part of its popularity isn't caused by the aural and linguistic alignment of the presumed good and bad choices to the major political parties in this country. The word pack conjures up homonymic allusions to the G.O.P.'s pachyderm, while the word herd does define a large gathering of donkeys.
Note: While you may find a Google reference to a group of donkeys being called a pace of asses, this is not correct. Any group of equines is called a herd.