Yours is the same errant thinking that permeated the Jazz coaching staff under Sloan's out-of-his-azz regime, when many players in crucial positions were not given enough playing time to demonstrate whether they could play a position or not.
In this case, you are writing off the potential of a veteran player (as opposed to usually a youngin' that Sloan underdeveloped) with historically huge self motivation and a history of already improving substantially in keys areas such as shooting. And although I wasn't able to easily sample how much time the Paperboy actually played SF (given that the NBA game logs appear to be disabled), 82 games pegs his PT at SF at a mere 2%.
https://www.82games.com/1011/10UTA11.HTM#bypos
Which would put his SF slate last year at--wait for it--around (2% * 82 games * 48 minutes =) 78 minutes.
He wasn't doing great (82games puts his per-48 +/- at about -5, which was six points below his PF performance), but in Millsap's case, the huge upside in the versatility of playing PM at the 3 is worth the investment if he doesn't cause the Jazz to lose a lot of games in the meantime.
Because a frontcourt with a Jefferson who actually defends, a Favors who can actually post (which also will likely improve with more on-court time), and a Millsap (or Hayward or CJ) who can hold his own at the 3 is quite formidable.
As has mentioned before, using Millsap at SF in special situations (e.g., wearing down World Metta Peace) is worth investigating anyway.