Is that really accurate? My observation has been that one tends to use it on behalf of the group for which one is advocating in an argument, but I wouldn't want to rephrase the first half of this sentence using the phrase "I tend," because I'm not referring to myself, but to every instance I have ever heard or read in which the case was used.
(Rereading this before submission, I realize that you could just quote the above paragraph and respond with "QED." :) More meat follows below.)
I assume that that sort of observation (that it coincides with an individual from the first person plural set) stems from the fact that one does not often pose arguments which prescribe the behavior of groups which exclude oneself - that doesn't mean it can't happen though.
One may believe that one's computer is powered by hamsters on exercise wheels, but one would be incorrect. :)