My LT has both arm rests and the passenger seat still folds flat as long as the height adjustment is set properly.

 

I posted complete pics on a center console mounted arm rest that I put in.
It both opens up and folds back.

https://www.chevyhhr.net/forums/hhr-ss-45/has-anyone-checked-out-ss-5-speed-yet-26334/

 

You are correct, an armrest would work OK on the drivers side. I think the reason most of us added to the passenger side is that, at least for myself, the seating area is a little bit tight and I tend to use the passengers side armrest anyway. My wife has an LT and I use the one on that side when driving by myself. I just figured that it was more comfortable, and when I do have a passenger, they now have one.
If money where not an object, I would add one on both sides, but let's face it, adding one using factory parts is a little pricey....

 

The GM Performance drivers seat does not have the necessary mounting point that the passenger seat has for mounting the arm rest on it. This is in the 5-speed version only.
That is why I went with the console mounted arm rest/storage bin.

 

Yeah this is something I have really missed in the HHR SS an arm rest especially on long trips. Will it fit the 2010 HHR SS

 

Pappa Smurf brought up a good point, it won't work on the performance seats, but shiould fit on all other 2010 seats.