Heh... I saw Don Nelson put three 7-footers on the court at the same time, along with a 6-7 SF. So which one of the 7-footers was the 2-guard? Those oddball rotations make a lot of the usual position labels meaningless.
When Ginobili and Hill are on the floor at the same time, it's hard not to call Hill the PG because he is shorter. And it's hard not to call Manu the PG, because he is generally running the offense. (It was also hard to call Roger Mason a PG, even when he was running the offense, because he was so inept doing it.) Last season, Hill played 2276 total minutes vs. 2150 for Manu, but Manu recorded 370 AST while Hill had just 226. Like it or not, Manu was the backup PG.
A good stat to look at is AST / 36 minutes. Parker usually averages around 6.5 - 7, which is a little light for a top-tier NBA PG. (Guys like Kidd, Nash, Rondo, Paul, etc. average 9+) Ginobili usually averages around 4.5-4.8, but last season he averaged 6.2. To me that says that he spent more time in a PG capacity than normal. By comparison, Hill averaged 3.6 AST/36 last year.
And Temple? Even though he played well last season, he averaged just 2.2 AST/36 minutes. Those aren't PG stats. They aren't even adequate backup PG stats. (In three seasons with the Spurs, Jacques Vaughn averaged 6.8, 5.0, and 6.2 AST/36 off the bench.)
To be successful, the Spurs need their backup PG to put up around 6 AST/36minutes. Last season, that player was Ginobili. If Hill or Temple can step up their game to that level, it will make a huge difference to the team. If they can't, those guys will be 2 guards, plain and simple, with Manu being the backup PG again.