Let's say the party tank has picked up, and attuned to, a Shield of Missile Attraction, which states the following:
While holding this shield, you have resistance to damage from ranged
weapon attacks.Curse. This shield is cursed. Attuning to it curses you until you are
targeted by the remove curse spell or similar magic. Removing the
shield fails to end the curse on you. Whenever a ranged weapon attack
is made against a target within 10 feet of you, the curse causes you
to become the target instead.
It's noted that the "curse" portion of the shield does not require that the character carry the shield, and also does not force the character to carry it (such as some cursed weapons do). Consideration was given to having the curse removed, but it is decided to not do so, as being able to completely protect other, more squishy, party members from ranged weapon attacks, by simply standing nearby, was considered a net benefit.
The downside, however, is that a character is only resistant to ranged weapon attacks while holding the shield.
So the question is, where does the break point exist that the character should hold the shield (and gain the resistance to ranged weapons attacks) versus the alternative of holding their +2 shield instead (which doesn't require attunement)?
As a limitation to the question, limit the comparison against the ranged attacks that the Resistance is effective against.
Best Answer
Attunement favors the +N shield
Not only do you not need to carry it, you don't even need to stay attuned to it. The options for ending attunement are as follows:
The prohibition on ending attunement to a cursed item only applies to the last clause. Therefore, you could still end attunement by removing the shield of missile attraction from yourself by a distance of at least 100 feet for 24 hours, or by having someone else attune to it. (I suppose you could also die and get revivfied, though this is rather extreme and costly for anybody who's not a Zealot barbarian.) See also this question.
Since a +N shield does not require attunement, you could end attunement to the shield of missile attraction, keep the curse, and use the +N shield with zero attunement slots occupied by shields.
On the other hand, if you wanted to keep the damage reduction from the shield of missile attraction, the attunement rules state:
and therefore you would need to maintain attunement with the shield of missile attraction to keep the damage reduction. Thus, on top of competing with the +N shield, the damage reduction is also competing with any other magic item that you could use the attunement slot for.
The damage resistance is less valuable for barbarians
Speaking of barbarians:
Therefore, if you are a barbarian, you will benefit relatively less from the resistance to damage from ranged weapon attacks.