Revealing mistake: When the Rangers kick Oozemen's ass, the pink Ranger starts off with the "You guys make me sick sick sick -kick". While all Rangers are fighting, the blue one is just standing still next to the camera. You can see his shoe in the in the lower right corner waiting for his entrance. (00:22:30)
Revealing mistake: When the morphed Red and Black Ranger fight the ooze-men, the Red Ranger does a "tornado-kick" right before Adam says "rocky behind you." In the middle of the tornado-kick there was an obvious edit of the shot. The ooze-man suddenly is a few steps back and the camera seems to have repositioned. This bad cut was more noticeable in the VHS-release. (00:24:30)
Revealing mistake: When Tommy jumps off the first floor and is missile kicking the ooze man, there is one shot that is been sped up to make him look like he is kicking super fast. (00:25:40)
Revealing mistake: Just as Tommy throws his knife pay close attention to the upper left of the screen. You see a very quick glint from the wire used to support the knife in the shots of it flying on it's own. Slo-Mo may help but is not essential to spot this mistake. (00:26:45)
Revealing mistake: When the Rangers de-morph we see them looking at their hands and bodies. However, when the actual suit disappears, their hands, arm and even heads are in different places. This is really noticeable with Billy. His suited hand and unsuited hand, while the same hand, are about a foot apart. (00:27:20)
Revealing mistake: When the last 3 Tangu warriors fly off to find the Power Rangers, one of them knocks the wall of the cave, which visibly moves. (00:33:35)
Revealing mistake: When the Rangers are attacked by the Tangu Warriors, Aiesha summersaults to avoid one and slams into a boulder, which then wiggles like plastic. (00:36:15)
Revealing mistake: In the final Megazord-fight, Aisha (Yellow Ranger) uses her Bearzord against Hornitor, then she says "I've been hit hard." Between the two shots, the power-emblem medallion on her chest falls off. (01:13:10)
Revealing mistake: Ivan destroys the monorail and then precedes to walk through the wrecked part. However the leg of the monster passes straight through a piece of metal. (01:14:00)
Answer: There's never a "point" to including a weakspot, by definition they are a flaw in the design that is either unforeseen or unavoidable. Maybe they could not find a way round it when building the robot, or any solutions simply created other weakspots.