I hate blanket rules.
Too many exceptions. But generally speaking I would only add an Elf Cloak to a significantly important Hero, and then only if the points are available without sacrificing something more useful.There are ALWAYS exceptions. A friend challenged me to a Contest of Champions scenario once and was convinced his Uruk Hai crossbows were going to shred my "soft" Wood Elves. So I showed up with Legolas in Elf Cloak, Radagast, a Sentinel and the rest of the (very few) points sunk into WEs with Cloaks. Practically eliminated his crossbow threat and forced him to close to melee. Though I had much fewer models Legolas was able to easily get more kills before his Uruks broke me (rather quick at close range due to far to few WEs to fight effectively). Cheezy, but it was a match were I 'just had to do it' and was taken in good humor.
That's an extreme example, but you get it. Cloaks on regular troops are only viable if you have a specific plan for those troops that requires the cloaks rather than extra troops.