i get shocked when i see Odom or Marion play SF, and you can say that they are PF at least that play it quite regulary. I am not the biggest NBA dude, but at least in my "home" league we had some PF who play SF quite often. And also a bunch of player who could replace the Pointguard, even our NT runs our point with an SF for some time.
For example my PG have 9 ID, IS and 8 Reb , my backup Center have fllowing Guard skills and would be still a C when he had 3 OD more:
Jump Shot: strong Jump Range: respectable
Outside Def.: strong Handling: respectable
Driving: average Passing: strong
If you play BB shaq here on SF you get trouble, but there are player who have the skill set to play multiple position or maybe aren't detected on the position most of us will play them ;)
And for your example, in my eyes Tim Duncan had more the skillset as an C but he would be still remembered as one of the best PF maybe even the best PF. Is it unrealistic? Also i think Garnett was a SF at least for a stint(2001/02 i am sure), and played it from time to time. In germany we had many PF playing also SF, of my favorite team the two PF backups also play SF and the main 2 SF helps often at the point in the last season really unrealistic those reality ;)