while its popular to dualclass fighters..
a lvl 15-20 fighter is just plain awsome. It may not have the tanking power of a fighter/cleric, But as a master of weapontry, and just hands down bashing, its really hard to beat...
use your mage/cleric to cast a few buffs on him and then get back to doing what ever it is you wanna do with your mage/cleric and you will see that there is really no comparison.
People forget its a D&D, and its all about the group... a lvl 10 cleric and fighter are definatly better than a couple of multi-class combos. You are getting more attacks, higher lvl spells, better saves, better thaco.. basically better in every single way..
sure if you had 2 fighter/cleric mulit class combos, it could be pretty decent, casting double holds, maybe a couple summons, command,, buff yourselfs, ext.. But at the same time that all takes rounds. and in dnd more rounds usually means its better for the other guy.
Now if you just had 2 pure PC's, Your fighter is already engaging and hacking away, while your cleric tossed out a hold, depending on how the fight is going you can cast a buff or 2, maybe your fighter needs a quick heal, maybe not.
Basically, if you have a fighter/cleric it cant fight and cast at the same time.. all your basically doing is being a caster, which makes your fighter part useless, or your buffing, then fighting, and then stoping to fight to cast a heal or something durring combat...
one character hacking away, and the other casting, is definatly a better way to go if ya ask me..
thats not to say that i dont dual class, or multiclass ever.. But many times the singles are the way to go. even things like mage/theives.. many mage spells make the thief ablities useless.. things like knock, invisiblity, dimention door.. But at the same time, it can be fun to have a jack of all trades toon.. the always under powered but never outmatched, mage/cleric/theif..
basically my point is.. if you havent tried a party of a bunch of single classes.. give it a shot, you might be surprised on how good it really is....
fighter, cleric, mage, theif, paladin, druid... give it a try,, You might be very surprised how well it plays..