I think the downside to Enfeebling Strike is that its special ability rarely affects combat.
Let's compare the special abilities of each:
1) Bolstering Strike. Not always, but this often drops Wis Mod damage from multiple attacks on the Paladin per encounter. The weakness of the Paladin or any Defender is that he is up front and typically getting attacked more than most other PC. So, he tends to often run out of Healing Surges first in a party, even though he has so many (Lay on Hands contributes to this as well). This could easily save 2 or maybe 3 Healing surges per day.
Overall, this is a good ability to have. The downside is that once the Paladin has the temporary hit points, he cannot gain more from attacking with Bolstering Strike again until he gets hit.
2) Enfeebling Strike. Although it sounds good on the surface, it is the equivalent of one fewer attacks hitting every ten times the enemy swings. Regardless of opponent of the Enfeebled NPC, the outcome of its attack is typically unchanged 90% of the time. So, it might help once per Encounter (on average). It is only when the enemy has multiple attacks (possibly with OA) that the odds here improve at all.
Plus, it can only be done on a Marked enemy, so it is limited to one opponent at a time.
Overall, this is not that impressive when compared to other Paladin attacks. It has its uses, but they typically won't result in a significant gain on any given encounter.
3) Holy Strike. This is a very good power. The additional ability adds to damage every time the Paladin hits unlike Enfeebling Strike which may or may not affect combat in a given Encounter at all (or may affect an encounter many times, but usually doesn't). For ten Holy Strike successful attacks at +2 Wis damage each, that would be 20 extra points of damage whereas protecting PCs with Enfeebling Strike would typically result in a lot less than saving 20 points of damage in return (although it could stop a condition). At +3 or +4 (or more) Wisdom, it's even better.
Like Bolstering Strike, Holy Strike will often save Healing Surges. But, it does it by damaging foes faster, not by protecting the Paladin more.
4) Valiant Strike. Like Enfeebling Strike, this one is conditionally helpful. Unlike Enfeebling Strike, the amount this helps is variable. That means that it is real useful the more adjacent opponents the Paladin has. But since Holy Strike is so solid, it is reasonable to typically only use Valiant Strike with 3 or more opponents, just to increase the likelihood of doing damage. This is especially true with minions where the special abilities of Enfeebling Strike and Holy Strike are irrelevant. This is the Paladin anti-minion At-Will power. None of the others compare.
The main claim to fame that a Chaladin has is that his Mark typically does more damage than a Str Paladin. This means that he is slightly more sticky as a Defender which is good. However, it also means that he typically takes the two At-Will attack powers which are somewhat less useful overall.
Bolstering Strike is less useful due to the fact that it cannot typically be used to gain its special ability every round and Enfeebling Strike is less useful due to the fact that its special abiilty tends to only help out once per encounter.
The other problem with being a Chaladin is that the At-Will Basic Melee attacks are based on Str, not Cha. So, the Chaladin will not be fulfilling part of his role with OAs for opponents that try to leave his vicinity as well as a Str Paladin, nor when charging.
So, to be slightly stickier to one opponent at a time, a Chaladin has to take slightly less useful and potent At-Will powers and gives up a bit on Basic Attacks. This does not, however, say anything about Encounter or Daily powers, just how the At-Will powers match up between the two.
Btw, as PCs get higher levels, monster hit points increase a lot faster. Monster defenses also tend to increase by about 1 per level whereas PCs attacks do not (PCs have to gain magic items and use synergy bonuses to make up the difference). Because of this, 4E is an offensive (and healing) game instead of a defensive game. The Chaladin is a more defensive style than a Str Paladin, hence, by definition he will start to fall behind a bit unless the potency and utility of Cha based Encounter or Daily powers make up the difference.