It depends on how realistic that potential is. If you had a choice between a 99% Mutombo and a 0.0001% superstar, obviously you'd go with the (almost) safe bet. This case isn't as skewed, but I'm very skeptical about Rubio, again based only on what I've read. Part of the reason he has so much "potential" is that very few NBA executives know much about him. Rubio is a bigger mystery, but that doesn't necessarily mean he has a higher ceiling.
I was pretty impressed w/ Rubio in the Olympics considering his age, I think his potential is very real. I disagree with your assertion that NBA execs dont know much about him. From what Ive read (and the evidence seems to corroborate this) NBA teams are doing a much better job w/ Euro scouting since the Darko debacle.