Quote from Saber in Blue:
Well, I think Larva are the coolest just in the "coolness" factor, but the you-don't-have-to-freeze-them thing is really a strength is you think about it. Fighting in Metroid is about staying alive long enough to kill the other guy first, so with Larva, the fact that they are vulnerable to ice is a huge weakness. It doesn't matter that they take five missiles after that or that they are invulnerable to missiles while not frozen. You see, Larva really only take one battle-hit to kill; when they get frozen by a single Ice Beam shot, they cannot attack or move. The battle is really over at that point, and the five missiles are really just a formality. With practice a person can easily go through all nine of them in II without trying. The invulnerability to Ice in the later forms means that they take 5, 10, 20, and 30 battle-hits to take down, and they remain dangerous during that time.
What you say applies to when samus fights them, but I was talking about larvas being cool in general. Think about it. How many people have ice weapons? Since the answer is none, metroids are pretty much invulnerable in their larval state. On the other hand, if the army of today was to fight some of the higher stage metroids, they would kill them easily. And larvas look waaaaay cooler. I win, now and forever.