The reason we can rotate players so often (SAF went a league record of some mental amount of games without picking the same 11 recently) and still put a quality team out and win is partly down to the fact that we don't have a 'first 11'.
We just put out the team that is most likely to win on the day. If the opposition have a particularly tall/strong striker or two, we'll put O'Shea at right back to add some height, whereas if they have a midget, we'll put Rafael in because, despite being a lot shorter, he adds more pace to our back line and is better going forward.
If we're playing against the Arse, then we'll need a hard working ankle-nipper in midfield and at least one defensive central midfielder, cf. O'Shea in the FA cup, Anderson in previous seasons.
When we're playing a top side, we play on the counter with two pacy wingers and a quick striker. If we're playing an 'easier' team (for lack of a better term) then we'll go 4-4-2 with Berbatov playing, as he can unlock high defensive lines with his passing.
In Europe, Park is essential against teams in the knock-out rounds as his work-rate alone is of huge benefit to nullifying opponents attacks. This was best seen in the 07-08 semis against Barca where Park was our best player, keeping Messi quiet for a lot of it. Domestically, Park is not as vital in most games, though he's still a fantastic squad player.
Horses for courses, 'n all that.