Saw this in its first release and thought it a little too extreme. But it sparked in me an interest in the Arthurian legends which continues to this day. At first I thought having Percival taking on the actions that are traditionally handled by Galahad was just Boorman looking for a way to shorten the narrative (as you say, it covers the entire legend - so you'd have to make cuts somewhere), but later learned that the Percival story is an old variant. This made me realize that classic lore is always being recreated and expanded upon; it's not just a story made up and handed down verbatim over the ages.