When Washington spoke of "parties," he was speaking from an 18th century British perspective. The British parties of the time were more what we would call "factions." With few real ideological differences, they were more struggles between groups of the British aristocracy for control of the goodies of government. No wonder Washington opposed them.
Parties in our modern sense developed in the US very naturally once real ideological disagreements arose between the followers of Hamilton and those of Jefferson.
If those who support expanding the role of government groupt together to push their policies, those who oppose this expansion must do the same if they are not to be overwhelmed. Parties are an inevitable part of our system now, which doesn't mean they don't have unpleasant side effects.
I think our parties today more closely resemble the 18th century British ‘factions’ of which you speak.
Sure, they emphasize their differences, in order to draw support. But look at what they actually do.
"Parties are an inevitable part of our system now, which doesn't mean they don't have unpleasant side effects."
One of those more "unpleasant side effects" is that BOTH major parties have been co-opted by the SAME GROUP of insiders, who are now working both parties toward the SAME GOALS:
Namely, the destruction of our national soverignty, and the submersion of our country into a one-world socialist government.
While the demoncrats will gennerally applaud that scenario, most Republicans seem to simply accept it as either "inevitable" or "must be o.k. if our Republican leaders are doing it", which serves to illustrate an overwhelming lack of understanding by the general populace.