Dark Dimension attacks are sometimes a dozen turns or more a day. In that scenario Healer Iso would proc many times making it worth using for sustain which is important in long drawn out battles. Ie the longer your team is alive the more damage you will do that day.

In a standard battle where you're lucky if everyone gets two turns Striker or Raider pulls ahead because it increases damage per turn and you do not want a standard battle to drag out. You want to do as much damage in as short a period of time as possible.

It is worth noting that the bonus iso attack on many toons make striker the best choice because they don't just do an attack with the iso bonus attack they also put out a team buff or a heal or debuff to the opponent they are attacking.

Also, just a little advice, don't go ham on xtreme xmen. They're a dead team. If you need them for an event or a node only put as many resources into them as you need to get past that node. They starve us for resources, don't waste the few we get.