only build i would even think to invest 10 into leadership into would be a build with 10 damage reflec as well since i suspect that even later on into that game most classes want those 10 points.
I could see a tank dropping 10 points on leadership even without damage reflect (which probably needs to be toned down anyway). Even 6 points in leadership cuts incoming damage on allies by a third. Again, such powerful effects need to be played around, not granted willy nilly. No, you're probably not going to be protecting all three allies the whole combat. But if you get back to, say, your wizard and ranger who are not too far apart, then they're going to take a lot more punishment from spellcasters and dodge a lot of attacks. Or if you coordinate with your rogue, you can let him wail on your enemy a little longer even though someone else might be targeting him.
And of course leadership will be super potent on all range or all melee teams, or even 3 and 1. Again, 5m is just painfully short, but 8-10m is the sweetspot.