It appears to me that utilizing the primer is the way to go and I don't understand why you would not want to do so. If nothing else, it will give you peace of mind knowing it was done proper.
Using primer isn't a big deal for me. I have the cleaner, primer, and cement on hand. But, I was just curious since I've never seen condensation piping plumbed with primer. Maybe the installers are going the cheap route.
In any regard, I bought one of these this past week and will be installing it. Current trap is clogged, so I will need to replace it.
One interesting thing though.
I currently have 2 zones and my attic unit has this clear pvc trap installed by the previous homewoner, but on the actual condensate line, not the drain pan as the instructions indicate. The installer modified the installation such that, as best I can recall, the T-fitting before the trap is open to the atmosphere.
Does the condensate line draw air in?
I remember going to big box stores and seeing the condensate lines that they carry, the dip is not as prevalent such as a trap for a sink, because a condensate line is not keeping gases out. I think, and correct me if I"m wrong, that a condensate trap is never full, but allows both water and air to pass in case of "drawback" or negative pressure in the line.