Don't quote me and make you case that its doable and then list 4 classes that can interrupt. I Should have specified merc ranged bc you are right. Sorcs have an interrupt.
My group yesterday didn't interrupt the plasma arcs either (I was the only one doing so, and that was mostly because I wanted to make it marginally easier - nobody else was bothering and it isn't the strat the guild I ran with uses). But really there are multiple ways to approach it, and you can easily make up for having a merc that can't interrupt. The only thing we interrupted was incinerate, and that was just the tank and the marauder.
For the record we wiped twice and we did it with:
If you're in a group where all 4 members can interrupt, then by all means go ahead and set up an interrupt rotation on the plasma arcs and the incinerate as well. Or you can do what we did and just kite around the room and avoid the plasma arcs. You'll have to run through the arcs and lava a few times while kiting, but as long as you don't stay in them it's nothing you shouldn't be able to handle.
The incinerate dots can also be cleansed. So that's another option if you want to focus on arcs instead of incinerate, but it's more work for the healer.
And it's not like it's a cakewalk for range dps if you're kiting him, either - I had my fair share of running through lava or plasma arcs as I had to get line of sight or within range while avoiding the mechanics. But again, if you're quick enough it should be well within your ability to take the hits.
It's definitely a harder fight than what we have in other flashpoints, but it's far from impossible regardless of class makeup. Sitting down with a cool head and figuring out a strategy is much more productive than simply throwing your hands up and saying that it's not worth bothering without a particular group composition.