The Shadow said:
All right, then try this on for size:
Ftr5/Wiz5 has martial 7, arcane 7.
Compare:
Fighter 10 (single class) has BAB +10 and 7 bonus feats
Wizard 10 (single class) has 5th level spells at caster level 10 and 2 bonus feats
Fighter 5/Wizard 5 (multiclass) has 4th level spells at caster level 7* and 1 bonus wizard feat, plus BAB +7 and 4 bonus feats.
You either lose 3 points of BAB and 3 feats, or 1 level of spells, 3 caster levels, and 1 feat.
Ftr4/Wiz4/Clr2 has martial 7, arcane 7, divine 6
Using the straight fighter 10 and wizard 10 from above:
Cleric 10 has BAB +7, 5th level spells and turns undead as a 10th level cleric
Ftr4/Wiz4/Clr2 has BAB +7, 4 bonus fighter feats, 4th level wizard spells cast at CL 7*, no bonus wizard feats, 3rd level cleric spells cast at CL6*, and turns undead as a 2nd level cleric
* If we're taking Bo9S as a starting point, spellcasters will use their effective caster level to determine the highest level of spell they can choose. However, they would only gain new spells known and spell slots when actually taking a level of their spellcasting classes. This would require some sort of retrofit of the system, which would work better if there were some kind of level-independent "spells readied" mechanic. In any case, a Ftr5/Wiz5 should be able to ready the same number of spells as a Wiz5, but should be able to choose 4th level spells as part of her repertoire.
Also note that in order to cast arcane spells, none of the above multiclass characters may wear armour.
I'm not making a judgement here. I'm just trying to spell it out so that there's a basis for comparison.
edit: Of course, I'm making these comparisons based on 3.5 assumptions. I think we can assume that if multiclassing rules in 4e operate on an "add 1/2 your other levels" basis, the classes will be designed to accommodate that.