1. Transforming mid battle is super unreliable, just due to how laggy it is.
2. In Melee the only real to transform to Zelda from Sheik was to recover, and even then you don;t need it all the time. Brawl is pretty much the same, even though they tried to nerf Sheik in Brawl to try and make transformation get used more, but it is still unreliable.
3. this finally gives Zelda a chance to be, at least, a decent character now that she isn't tied down and let sheik go back to normal, now that she isn't tied down to being bad for a transformation, and possibly be top tier again.
4. The characters already have tools that make them full characters even with transformation, just Zelda has always had bad tools while Sheik's tools got nerfed in Brawl.
5. Transformations have always been not good. Deal with it.