Two half-hearted counterarguments -
1) Generalists only win in times of crisis and change, when the support structure that allows the spec* to strive and advance is disrupted. Most of the time, the spec* wins.
2) Most sci-fi stories where the generalist ones would probably be ones where there's a lone hero, who has to do everything on his own. An rpg, on the other hand, has a group of protagonists. Do you want them all to be generalists?
*: insert an ialist here. The spamfilter thinks I'm trying to sell you drugs.