Leaving the last roster spot available is actually not a bad idea; in the past the Spurs have used it to make some additions or roster changes right around the trade or free agent deadline. Roster Flexibility is a good thing. And in my opinion, that option is mostly needed by the Spurs this year. Mainly because we haven't seen how the current roster will play out. So if things don't go well (crossed fingers), having a free roster spot certainly helps.