I think that ability would work great for Haldir in Rohan's summon, because that power is currently a little weak. However, if that is added, there really isn't a reason not to use it, so I would suggest not allowing Haldir or the Galadhrim to switch to their swords while the ability is active, making it somewhat of a risk.
And maybe this would also be a cool ability for the Caras Galadhon Guardians, who in my opinion need an ability like that. However, that the Caras Galadhon Guardians need to get another ability, because if I remember correctly, they start with the other 2 abilities they have. I don't think that Lothlorien's Haldir should get this ability though.
On a seprate note, I do have a suggestion for changing one of Haldir's abilities a little. I do like Haldir's last ability where he summons the Marchwardens (I think that's what they are called. Correct me if I'm wrong). But I think it could be slightly changed to where if Haldir is using his bow, the Marchwardens he summons will also be using bows instead of swords. And it would stay as it is now if Haldir is using his sword.
I think it would make more sense because I would believe the Marchwardens, just like Haldir, would be trained to use both the Bow and the Sword. And depending on what weapon Haldir is using, he can choose what the Marchwardens specialize in. We also see them with their bows in the movies when they ambush the Fellowship, so I would assume it is lore accurate, though I don't know too much lore past the Movies so I might be wrong on that.