Storm Raven said:
No it is not. It is a trivial nuisance at best. Have your party Wizard, Sorcerer, Bard or Cleric spend a mid level spell slot to cast Greater Magic Weapon on 50 arrows before you go into battle. By the time you get to a high enough level that you are dealing with significant foes who have damage reduction, this is a trivial cost in spellcasting resources, and the arrows cost no more than normal arrows.
Oh, so the party has to have one or more of these classes (or Paladin) if it has an archer type?
A Sorcerer wants to take GMW instead of Fly, or Haste, or Fireball, or Dispel Magic as one of his four 3rd level spells? Just to help out the party archer?
A Bard wants to take GMW instead of one of his five 3rd level spells? Just to help out the party archer? Maybe if he is an archer type himself, but otherwise…
The Paladin cannot do it until level 11 at the earliest. Even then, they are +1 arrows. +2 arrows at level 12, +3 arrows at level 18. Not very helpful against Stoneskin or powerful demons at equivalent levels.
That leaves Wizards and Clerics. Most of the Wizards I know would rather memorize a Fireball or Haste or Slow or something that does more than a few attacks per round. Especially since in non-DR circumstances, magic arrows will only do an extra point or two of damage if they hit. Third level spells are especially precious for Wizards.
A Cleric taking GMW is not taking Neutralize Poison or Restoration or something equally helpful in the areas where mostly only Clerics can help. Yes, the archer helped kill the Wyverns, but not before they poisoned 4 characters, 2 of whom died.
Yes, having an ally take GMW COULD happen and probably does in higher level campaigns. But, unless you know ahead of time you are running into DR creatures that this can even affect before preparing in the morning, it probably will not happen most days in most mid or lower level campaigns.
Plus, casting it the morning for a morning encounter will probably not help much late that night due to duration without the Extend feat (even higher level spell) or multiple castings.
Mostly, archers are on their own to acquire magic arrows however they do it.