Why the R1 over the R2?

For me the R1 is just warm (or cool) enough to wear while active in cold weather but where as I would cook in the R2.

Hope that helps.

+1 to above. R2 is just too hot for me to wear when moving.

Same here, I find the R2 just too warm when moving.

Okay, I can see that, but it does have a zipper.

Clearly you have not used an R1. It is a near-perfect insulation value. Anything heavier won't be used much in the lower 48.

I am wearing one at work right now as it is smart enough to wear over a short sleeve shirt.

+3? to above. R2 is just too hot for me to wear when moving.

I have found that I can get a much larger range out of the R2. Part of this may have to do with the fact that the terrain around here is flat, there always seems to be a slight breeze which favors the looser weave of the R2 and I have thin arms. I found I was able to wear the R1 at about 5-7F higher temps than the R2 because of its thinner material but I was able to go much lower with the R2 while moving or stopped because the heat properties of the R2 are much more than the R1 (roughly 0.30 vs 0.76 Iclo). Especially with a shell which really takes advantage of the R2. But then again I don't get too overheated wearing an R2 below 50F if I have it fully unzipped with just a non-wool poly base layer and can usually push it below freezing which I can't do with an R1. But then again pretty much all of the hiking I do has almost no exertion involved so it is probably just me.

And I couldn't even wear an R1 to work over a T as I used to roast in an R0.5 when I owned some when going to work and the temp was above 65F.

I also tried out an RAB Alpine Generator for a few hours and found it only to be slightly warmer than my R2 and a shell which I thought was weird.

Hiking on the flat requires far less energy and thus generating far less heat.