It's not strictly match up related. The question isn't necessarily, "What match up would be worth switching for" (although, some are so bad, it's worth just playing one form the entire time), but rather, "What instance is it worth switching for." Individual match ups play a role, sure. For example, I feel like if your Zelda and Sheik are the same level, you can get away with going Zelda the entire time against Samus. Should you though? Probably not.
The more important dynamic to look at is how Sheik/Zelda, in tandem, counter character play styles and fill niche weaknesses the other has. For example, let's say you're playing against Samus as Sheik. You rack up a moderate amount of damage, put Samus in the air, and you fade back, giving her room to come down. As she's coming down, you switch to Zelda. Now, Samus' space doesn't really mean much, because Nayru's negates missiles, and since it's a reflector, and not a power shield, 18% high knockback missiles coming back at you really slow down your projectile wall. So Instead, Samus will try to space in the neutral, close the ground. It works for a bit, but eventually as Zelda, you land a kick, and now Samus is far off stage. We all know Samus has a slow recovery, so you telecancel over to the ledge, and transform. Now you're edge guarding becomes significantly more potent as Sheik. Needles destroy her bomb jumps, gimp her tether, and hurt her up+b. You're pretty much dead at this point and free to transform to counter the style the Samus player displayed in the first stock.
This is kind of a rough example, but the point I'm trying to convey is that when used properly, in tandem, they greatly cover each other's weaknesses. It's knowing how to use their individual strengths, and not just playing an entirely different match up.
For the record, I don't expect you to change your opinion, and I'm sure you don't expect me to change mine. I'm happy to just provide a discussion.
Sure, and that's definitely true. You should really know all match ups when it comes down to it. But why do you get to select two viable characters at once, but I can only pick one?