I'm pretty sure he didn't sing that part in Reno, but did sing it afterwards when I saw them at the Nokia theater or US airways center if not both...I could be wrong but I'm sure I've heard him sing that part at least once. It's like when he wouldn't sing "Don't just call me pacifist.." part of Aenema in 06 (http://www.youtube.com/watch?v=1mPyWFunf3g) but then started to sing it again after that.
I don't know why he would choose not to sing those parts at times though, and it definitely does retract from the songs
I never noticed that he stopped singing that part in Aenema. He used to sing that last part of Vicarious when they first started playing it but I think he stopped singing it after the "official" tour was over.