It ruins team play if you have to stay out of each others way the whole time. What a stupid standard.
Scrub Alert.
Team Attack On teaches a balance of control and trust between your team mate and you.
It helps support real teamwork and viable winning strategies, as well as void alot of really broken and completely unacceptable spam moves, which ruins the doubles metagame.
With Team Attack off, you can't save your opponent from off stage if they can't recover, you can use G&W's bucket, you can't do a lot of really effective strategies that involves physical contact with your partner.
You will have to watch out for their attacks, but if you're a good player, that shouldn't be a worry, because you'll be handling your current foe and jumping in to help your team mate when they need it.
It also helps when you're the last remaining team member left, if your opponent is down for the count, would you like two Falco's laserlocking you with no effort? Or a wild barrage of Snake tilts or Mach Tornadoes? They can spam moves twice as much because it garners no consequence.
Overall Team Attack on is the most balanced form of doubles, it is an accepted approach to team play, and really only noobs, spammers and scrubs complain about it.
Just turn it off. I've never seen someone nit pick about a small feature in Brawl+ before.