The reason for this is that one of the fundamental lessons of D&D still holds true - you win by killing them faster than they kill you, healing doesn't keep up.
I have to disagree with this. In our 3.5 game, the most prized items for our party are healing items, not damage dealing items. In 4e, PC's die when they run out of healing. Also, the larger the group, the more healing they will need. So in a party of 3 or 4, a Warlord as the only healer may work fine. But in a party of 5 or 6, if your only healer is a Tactical Warlord, you are either in deep trouble, or the Tactical Warlord must resort to picking up some Inspiring Warlord abilities. The other option is to accept that you will have the occasional character death. A Paladin may be able to reasonably support a Warlord in a larger party. But a Lazor Cleric will still be way better at keeping the party alive longer.
If you are looking for a Leader for a party of 6, my vote goes to the Lazor Cleric. If you are looking for a Leader for a party of 4, my vote goes to the Inspiring Warlord. For a party of 5, it depends on whether there is a Paladin or not, but usually, I'll prefer the Lazor Cleric.