So I am reading Adrian Goldsworthy's book Roman Warfare and currently on the chapter where he describes the early roman triplex acies in detail. He mentions how that roman heavy infantry often carried two pilum, one of which was significantly heavier than the other. I was under the impression that a legionnaire threw both pilum as precursor javelin before switching to their gladius. If pilum were designed to bend once they made contact with an enemy shield to render it useless what would be the point of having one being made heavier than the latter?
The weight of throwing spears is a tradeoff, with the limiting constant being the strength of the human arm that makes the throw. Heavier spears hit with more momentum (and are, therefore, more likely to penetrate and damage what they hit), but the tradeoff is that you can't throw them as far. Lighter spears have more range, but have less stopping power.
Giving a soldier two spears, a light and a heavy, allows you to benefit from both sides of that equation. While the battle lines are far apart, you throw the light spears. When they get closer and range isn't a problem anymore, you throw the heavy one that will be more likely to penetrate enemy shields (where it will, as you say, become stuck and encumbering) or, ideally, injure the enemies themselves.