I think the quote was "That armor's too strong for blasters" implying that most of the body can withstand lots of blasts. This is similar to current day tanks which can take lots of punishment from small arms fire but can still be penetrated by lucky shots in one of its many vulnerable parts, such as the turret swivel, tracks and sensors. Being in a tank while it is being shot at by a machine gun is very distracting.
There is a scene where a blast from someone in the trench shot at, and hit, a rear leg joint causing a large (relatively speaking) explosion.
Another point I remember from my Marine Corp days is that having as many guns firing at the same time has an effect of causing attackers to concentrate fire towards your main fortification so that a flanking attack has a better chance of success. The flanking attack by the snow speeders in this case.
This particular battle is also a rear guard action, so killing is not the objective, just slowing down the attack. For that purpose, the more guns firing, the better.
All in all, this scene looks like the first encounter German Forces had with English tanks in WWI. Machine guns were fired at the tanks and managed to stop a lot of them.