Being evasive in teams is ineffective. If you have an opponent doing that, then just ignore that particular player, and double team the partner. They'll soon realize how futile such a tactic can be.
^^^^ Pretty much this.
I know how annoying people running can be. I really, really do. A friend and I run into that stuff all the time in 2V2s because they either want it to be a Sudden Death, or avoid a Sudden Death. I will say though, that sometimes it doesn't really work in their favor at all.
Someone can have 127% damage and run all they want for the last 30 seconds. They can go ahead and stay out of it all they want; however eventually they'll have to come to their partners aid. Then again, I've literally had people watch us wail on their partner and do absolutely nothing to come to their rescue all because they didn't want to get KO'ed. That didn't help them at all and allowed us to get an easy KO that way. I personally don't understand why people do this but whatever floats their boats I guess? I'm not going to complain.
I also find people who like to stay at the edges and spam attacks in 2V2 while their partner does all the work. That's fine by us. Again it makes it all the more easier to kill their partner when they're literally doing nothing to help. I must admit though sometimes those types require the most attention, such as Robin or Samus cause we can't allow them to full charge their attacks and spam them.
That would be bad. Annyyywaayyys...
Personally I would like both to be an option if it were possible. Either or works fine to me but I feel like Stock Matches would offer a little more of a challenge sometimes. It will force you to better protect your partner and/or work together or it may cost you.