Another thing people should consider is that Zelda and Sheik can be used at different percents. Consider Sheik vs Roy when both characters are at 0%. Sheik's going to take a lot of damage if Roy gets a hit, and Roy can CC through a lot of Sheik's moves at this percent. Thus, from a "Sheik is the better character" perspective, Zelda can be used to wrack up damage until it's safe enough to switch to Sheik.
The other instance is that Sheik is basically dead when she hits 100%. Most characters will send her far enough off stage with a strong tilt/throw that she'll be easily edgeguarded if she does make it back. Zelda not only has a better recovery, but excels at getting poke damage/trades when an opponent is thirsty for the approach.
Overall, I think Zelda is a worse character than Sheik, and that Sheik is probably the better matchup for every single matchup. However, there are certain situations in every matchup where I think that Zelda excels over Sheik (high percent, recovering, killing off spaced aerials) that makes having a secondary Zelda definitely worth it.
It's a shame that her movement feels so BAD though.