In the last session, one player was down to zero healing surges and had only 2 hitpoints remaning at the end of the adventuring day.. his AC sucks. I think most of the monsters his him on a 5 or better. He is also a TWF Ranger and deals 'nuke' level at-will attacks... and has an absolute blast.
There is nothing wrong with this approach. The player is having a lot of fun and fun is the name of the game.
However, this type of approach will not work forever and might eventually result in a TPK.
The reason is probability. There are just times when the DM's dice are hot and the Player's dice are cold. In this scenario, the DM's dice are even hotter for this one Ranger due to the fact that even a lousy roll hits.
So, this Ranger will get hit a lot, the Cleric will run out of heals, this Ranger will go down and it's the start of a downhill slide. And when this happens, a DM should not fudge the dice rolls. He should just let it happen. The reason is that it is not fair to the cautious player (or the other players) to have the DM protecting the PC of the risk inducing player. The DM should be impartial.
The problem with having low defenses is the swinginess of combat. The purpose of Daily powers is to attempt to offset this swinginess when either the encounter is going against the PCs, or when the encounter is more difficult than usual. Having low defenses will result in getting hit a lot which in turn will result in other players playing their Daily powers.
The concept of low defenses combined with a PC that goes out of his way to get into harms way will result in the other players being quasi-forced to use resources that they may not have generally decided to use. This behavior can also reduce the number of encounters before an extended rest.
As a DM, you also have to wonder if the player of the Rogue is being more cautious than he might otherwise be because he knows that the Cleric will be focusing healing on the Ranger and that there might not be that much healing left over if he gets into trouble as well.