I havent seen the teaser yet because my laptop sucks and videos play really slowly, plus i am among the unwashed masses who havent seen cloverfield BUT...
as described, it sounds like the point of having nimoy deliver the line is that most fans of trek and people familiar with the series would recognize his voice immediatley and get excited about the movie. Maybe i'm in the minority but ive never heard pine's voice before so I wouldnt know what it sounds like. Nimoy lends instant recognition because hes part of the trek legacy...pine and quinto, et al are just starting out down that path.
Now, it would be cool if at the END of the movie, if it indeed depicts Kirk going from zero to hero, Pine did the voice over.
Plus nimoy has a super cool voice.
Bring the awesome