You'd probably have quite a functional group if you replaced fighter types with clerics. However there are some weaknesses you'd have to compensate for.
1. Damage dealing capacity. Fighter's don't have to worry about wisdom so they can make strength a higher priority in stat selection. Fighters also have more feats--including weapons specialization (babarians have rage which increases attack and damage). Finally, fighters have higher attack bonusses, which, combined with power attack will usually enable them to deal even more damage.
Compensation: Divine Might will help high charisma clerics deal damage (only in a high stat game though--otherwise you're better off putting the high stat in strength so you can have good damage all the time). Divine Favor, Bull's Strength, Divine Power, and Righteous Might will all help even the damage dealing capacity.
2. Damage soaking capacity. So the one or two hp/level difference between a fighter and a cleric isn't much. It will add up though. Furthermore, fighter types can afford to have higher armor classes than clerics. Sure they both wear the same armor but a fighter typically has a higher dexterity. Clerics also need a free hand to cast spells--this typically precludes the use of shields. Especially when the enemy needs 16s or higher to hit, two points of AC can amount to a 25% difference in the amount of damage taken.
Compensation: Magic Vestment, Shield of Faith, and Endurance will help even things up.
3. Power up time. A fighter type starts dealing damage in round 1. In order to equal the fighter, a cleric needs to start by casting buffing spells. By the time he's done buffing, the battle could very well have been over. Since many monsters dish out massive amounts of damage very quickly, this could be a critical difference. (You really want to take down trolls and girrallons, etc before they rend and not after). The party of clerics could end up even shorter on healing than the mixed party.
Compensation: Be willing to go into a fight with only long-term buffs up.
4. Versatility. The fighting classes can have as many as 18 feats or multiple rages and uncanny dodge, etc. Clerics will probably be lucky to complete two feat chains by 20th level. Furthermore, fighting classes have proficiency in all martial weapons. Clerics with the war domain are proficient in one martial weapon. When the party is fighting a flyby attacking creature, the difference between the fighter's backup mighty [+4] composite longbow and the cleric's light crossbow is immense. If the fighter has weapon focus, and some archery feats, the difference becomes even larger.
Compensation: Clerics can compensate by taking ranged attack spells (produce flame if they have fire domain, searing light, etc.) Clerics can also use spells (like Zeal from DotF) to substitute for feats (Zeal would be a fairly good substitute for mobility in some situations).
On the whole, I'm pretty sure that a party with clerics replacing the fighting classes could work but it would take some skillful character construction. Given that, it would probably be at least as effective in a situation with time to prepare but less effective than a standard party in an unexpected combat.