It's because unless you have that SV quick-switch glitch (that's fun to say,
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
), switching is going to be tedious. You will almost always have to end up fighting with at least two pokemon, and since hacks aren't going to be universally applied in rulesets, there's no way to play standalone pokemon. Pokemon also get fatigued, so even when you don't die, you still have to switch because your pokemon get really pathetic at killing (Iirc Squirtle actually gets some cool stringing ability from fatigue, but that's it).
Sheik/Zelda is pretty convenient, conversely. You have the ability to go through an entire match and not be penalized for not switching, and have the benefit of having a few MUs lessened in severity for Sheik (like Pika, ICs, and D3). That being said, the MUs are still bad usually (except for D3, Zelda's deceptively useful in that MU), it's just that Sheik doesn't have to worry about getting CG'd to oblivion (and Zelda has a couple of gimmicks in those MUs).