I've been thinking on this, and have to say that I'd disagree.
Firstly, adding an ECL to something is effectively an experience penalty.
ECL additions should be, IMO, reserved for uncommon things (which are usually quite powerful). Clerics are common. Drow (at least on the surface or in Sanctuary), genasi, asimaar, tieflings, halfdragons and all that are uncommon. All these races have unique, special powers over the standard races, which take form in a variety of things:
For example-
Magic resistance
Ability bonuses
Spells
Inherent powers
Immunities
Clerics, while certainly powerful, are a common class. Of course, any mortal who can channel the divine power of the gods is defintely more powerful, ICly wise, than most.
However.
Some fighters train and learn so much they can kill almost instantly. Some bards learn to inspire allies to uncommonly great deeds. Some rogues become so sneaky or so precise with their blade that they can kill at will without having to think of beings caught. Then there are mages. I dont need to say how powerful they can become.
Each class has their own skills. If played well, any class can become powerful. Granted, clerics are pretty damn powerful, and more domains to select from will no doubt increase this.
But I dont think they should incur any experience penalties.
Unlike a fighter, or a rogue, a cleric has an IG responsibilty to uphold and indeed spread the dogma of their patron. Becuase of this responsibility, clerics *usually*, IMO, are more exposed to PvP hostility and fighting. Power comes with responsibility, and responsibility has consequences.
Others beleive mages to be the most powerful class...what then, another debate on whether they should accrue an experience penalty? What if others lobby that monks are too overpowered?
Again, this is my own opinion only, and does not in anyway represent the thoughts of others.