I tend to look at the assault on the bunker on Endor's moon as being very believable really. They even show physically how the "damage" occured.
Like the opening "volley" from the Woks showed that the arrows were hitting some chinks in the armor... The armpit, crotch, etc... There's weaknesses there for arrows to have an impact, especially if the Woks are accurate. Native Americans were NOTORIOUSLY accurate with their bows, some capable of a throat shot from horseback, so there's some possibilities there with projectile weapons.
Also note that when the EMpire is "stung" they bite back right away and the Ewoks show they have weaknesses too... They're uncoordinated somewhat, the blaster weapons show their damage tremendously... The Ewoks were maybe feining the weakness somewhat to draw the walkers out to their traps (smart move on the Ewoks part if this is the case), but the Empire definitely had reason to fear the Ewoks. They were seemingly outnumbered tremendously too, and surrounded, which no matter how technologically inferior, that's a bad situation to be in (again citing Native American battles with whites, such as the Battle of Little Big Horn, where whites were tremendously outnumbered and outclassed by the natives).
As far as melee weapons and armor... I said this once at another forum about this discussion, but basically my comment was to throw a helmet on, like a motorcycle helmet, and let someone smash you on the head with a stone axe or some similar weapon.
I bet you are at least dazed, if not actually having your skull broken open to some extent and go unconcious. It's amazing how easily a blunt object can incapacitate a person, much less a flurry of many blunt objects thumping someone (even armored) in the body/head.
In the Middle-Ages, as mail armor was phased out for plate armor, the plate armor had to be overcome. Arrows didn't puncture it well except at the joints... blades glanced off it generally, so the blade was actually not so much for puncturing as it was for crushing. Thus weapons like maces and cudgels became much more prized. The pike/pole-arm would become one of the most devastating weapons to knights because it featured usually all the tools needed to open those living tin-cans. A hook to dismount the knight, a "spear" tip to puncture at chinks in the armor, and an axe-blade to crush/chop at the armor.
In general though, "crushing" bone became more important than getting through that armor, and I think that would applie to the Stormtrooper armor.
Stormtrooper is a term coined during World War I actually (for those curious). The STormtrooper was actually an elite trooper trained to quickly attack an enemy emplacement, take over, and hold it till reinforcements arrive. They were the original "Blitzkrieg" really. They were specially trained, had special gear (some homemade actually), and were skilled in close-quarters combat since they were basically in trenches fighting with the enemy. They were also known as "trench raiders". During WW1 they had armor chest plates for that close-quarters battling and helmets of course, but these things didn't stop bullets or all forms of hand weapons.
The Stormtroopers DID carry cudgels though, used their spade as a weapon, carried trench knives, and usually a handgun over a rifle... They meant to crush and incapacitate enemies quickly, as did their enemies intend to do to them, and a spade smashed off that chest armor probably would at least take the wind out of the guy... Or smashed over the helmet or whatnot.
Anyway, long-winded post there.
The end result is that armor is good for protection against glancing blows from melee weapons, a glancing shot perhaps from a blaster, but in the end it really doesn't offer you a ton of protection. It's like Kevlar for our troops in the field today, or up-armor kits for our military vehicles. They MAY save a life, but there's no way they'll always (or at times even often depending on weapons and circumstances) protect the wearer or vehicle. It's not possible (yet) really.