Background: Servant of the Secret Fire, as published in English, gave +1 to combat and leader rerolls. In the Master Docs, this was (accidentally?) moderated down to just combat rolls, making the card about as useful than Devilry of Orthanc. Even though the combat effect is meek, the card is still very difficult to get into play, as Gandalf has to be in the battle, which can be very dangerous to the Free People's dice supply.
It occurs to me that Gandalf tended towards being messenger and diplomat. It was Aragorn who is seen as the natural commander at the big battles, e.g. Cormallen. Helm's Deep.
Anduriel works almost equally well with a small army. S/SF is basically worthless with small armies. Thematically, I think the designers got Anduriel's effect and Servant of the Secret Fire's effect reversed.
Gandalf would have had a difficult time being commander at Helm's Deep, since he wasn't there. He was basically the commander at the siege of Minas Tirith, and Aragorn was not there. I think S/SF should definitely apply to both combat and leader rolls. Keeping in mind that Gandalf won't have his own leader roll if he's holding off the Nazgul, but his presence should inspire the more mundane leaders.