Team attack on removes the skill-less aspect of not truly needing to coordinate and "look out for" your partner, while at the same time opening up a myriad of techniques, such as:

-Healing Ness/Lucas
-Filling Game & Watch's poop bucket.
-Having a partner feign a missed smash and caping them around.
-Having your partner feign a would-be-whiffed smash and then FLUDD/Water-gunning them into range of the enemy.

It's just more fun to have team-attack on, unless your partner is an Ike/DK/Bowser/Snake/Dedede who just wants to throw out Forward Smashes in the middle of the fray at any given time.

My only question is, if you do call a character after the first fight do you have to use only that character even though they can switch mid-battle?

So it's like you have to call Zelda and you can't use Shiek even though going back and forth is the point to that character.

Same goes for the Pokemon Trainer, If that's the case then the character choosing rule is stupid. IMO

The only person I can somewhat understand is Samus and ZSS but that's because once you switch mid-battle you can't go back without the smashball which we don't use lolz.

Nonono, Zelda-Shiek is one character, just as PT is one character.  Samus-ZSS is debatable, but IMO it's also just one.

Team attack on removes the skill-less aspect of not truly needing to coordinate and "look out for" your partner, while at the same time opening up a myriad of techniques, such as:

-Healing Ness/Lucas
-Filling Game & Watch's poop bucket.
-Having a partner feign a missed smash and caping them around.
-Having your partner feign a would-be-whiffed smash and then FLUDD/Water-gunning them into range of the enemy.

It's just more fun to have team-attack on, unless your partner is an Ike/DK/Bowser/Snake/Dedede who just wants to throw out Forward Smashes in the middle of the fray at any given time.