It's one of the most important factors if that's what you're looking for. If you're satisfied with getting a solid starter for a decade with the #9 pick (which I am), it isn't as important. If you want to take the risk on a project that could potentially be better in the long run, upside is important. Unfortunately, there aren't very many NBA ready players with a lot of upside, so you gotta pretty much pick one or the other. At the 9th pick, I'll take more NBA ready over a project with upside every time. But, that's just my opinion obviously...
But in my opinion, a player's upside should one of many factors. I think how a player fits in with a team and fills weaknesses should be most important, unless a special occasion arose (like if the Jazz landed the top pick in draft).