I like aw's localt0 hack [http://awwx.ws/localt0] but can't (yet) see any reason other than compromise not to allow reassignment of 't and the other symbols even at the global level.
He brings up an interesting point about 'nil though, reminding that it's equivalent to the empty list. I could be mistaken, but it seems as though Arc makes 'nil the more fundamental of the two. My evidence for this is that '() evaluate to nil at the REPL, rather than the reverse:
I say it should go the other way around, with 'nil evaluating to (). After all, if any symbol in Lisp is sacred it's parens, not the word nil. But more relevant to the point, this frees 'nil up to be a settable symbol that's only initially bound to the empty list.