Have a look at Koeman's record as a player and look where that got us.
Of course, being a great player doesn't make a great manager, and being a shite player doesn't make a shite manager.
However, the longevity of Gigg's career (at the top) suggests that the potential is there. A player cannot play at the top level for 20+ years without understanding the game, being disciplined, and having a good work ethic. I'm not sure that anyone has ever accused Barton of having any of those qualities.
Then there's the fact of playing all of those years under Ferguson. A student can study at Oxford or Cambridge for 20 years without learning anything, but at least they have been exposed to a great deal of wisdom.
I'm not saying that Giggs should be the next manager. However, I think he is easily distinguishable from Barton.