Why is it i feel everyone is describing a 2v1 match? You're not gonna be able to get math in the fray while Zelda launches side-B. In 2v2, generally speaking, both characters are always paired off fighting each other. Even if you could do what you have described Zelda's side-b is not that hard to dodge and, while annoying, will not amount to enough help to keep Marth from being slaughtered in his 2v1 match up.'
The most important thing about teams is the ability not to hit eachother, making Samus/Marth not always the best partner, and the ability to make the best of your oppertunities/stop your opponents from making the best. In other words the ability to make the most of the 2v1 time as one person is launched off the side and being able to distract your opponents when your teamate is being chain grabbed/ off the ledge. This is less important by far in Brawl as combos are few and recovery is generally uberfied, but someone with a nice fast spike to knockout an enemy chasing an ally off the ledge would be nice.