The plus bonus is so coveted because it applies to every attack, of which you will be making many throughout the game. In most games, the attack roll will be your single most common type of d20 roll, so adding a bonus to that will have a greater (cumulative) effect than adding the same bonus to any other type of check.
But arrows ignore the part about the plus that would actually make it useful or desirable in any way. No matter how many attacks you make, that +1 arrow will only ever modify a single d20 roll. If your attack misses, in spite of that +1 bonus, then the item was entirely worthless for you and you would have been better off had the thief replaced your magical arrow with a non-magical one. If your attack hits by a margin greater than 1, then the +1 bonus on the attack roll was entirely worthless but you still gain a ~10% boost to the damage roll (which, itself, only matters at all if the enemy is brought to exactly 0, and dealing one damage less would have left it alive and acting with 1hp).
So that's not my view of +1 arrows. It's a mathematical fact that a +1 arrows only meaningfully increase your efficacy ~5% of the time, and the rest of the time it either does nothing or increases your damage by a negligible amount.
