Full BAB, full spellcasting, d10, no spell failure?
No. BAB is irrelevant if you don't have the hp to use it. Spell Failure is irrelevant if you dont have enough spells to make it worthwhile.
The entire structure is built on EL/CL/PL (Party Level) and assumes a general party strength equal to about 1 fighter, 1 rogue, 1 cleric, and 1 wizard.
A party like the following:
Ftr5/Wiz5
Rog6/Sor4
Clr5/Wiz5
Sor10
Will not do as well as its "supposed" to given encounters in the CR system. 1-3rd level spells when encounters assume fourth and fifth are a good way to get a party killed. Compare it to:
Ftr1/Wiz5/Eldritch Knight 4
Rog3/Sor4/Arcane Trickster 3
Clr3/Wiz3/Mystic Theurge 4
Sor10
Is the second party "overpowered"? Its still the same theme, a heavily-arcane party, but which is going to be able to combat CR 10 encounters more easily? Lets break it down:
Ftr5/Wiz5
HP: 10+4d10+5d4 (avg 42.5)
BAB: +7
Spellcasting: Wiz5 (3rd level spells)
Bonus Feats: 4 (3 fighter, 1 wizard)
Ftr1/Wiz5/Elk4
HP: 10+5d4+4d6 (avg 36.5)
BAB: +7
Spellcasting: Wiz8 (4th level spells)
Bonus Feats: 3 (2 fighter, 1 wizard)
The ElK has 1 less feat and 6 less hp, but a lot more spells (Wiz5 vs Wiz8). Both have problems casting spells in armor, so both probably wear Bracers of Armor and fulfill more of a skirmisher-type melee role or an archer role.
Rog6/Sor4
HP: 6+5d6+4d4 (avg 33.5)
BAB: +6
Spellcasting: Sor4 (2nd level spells)
Sneak Attack: +3d6
Rog3/Sor4/ArT3
HP: 6+2d6+7d4 (avg 30.5)
BAB: +5
Spellcasting: Sor7 (3rd level spells)
Sneak Attack: +2d6
The ArT has 1 less sneak attack die, 1 less BAB, no Uncanny Dodge, and 3 less hp, but a lot more spells (sor4 vs sor7). Because it seems to lose a lot compared to the straight multiclass build (and because they didnt want to give the prc UDodge, Trap Sense, etc) especially skills, the arcane trickster has special things it can do 1/day, or eventually a few times per day.
Clr5/Wiz5
HP: 8+4d8+5d4 (avg 38.5)
BAB: +5
Spellcasting: Clr3 (2nd level spells), Wiz3 (2nd level spells)
Clr3/Wiz3/MyT4
HP: 8+2d8+7d4 (avg 34.5)
BAB: +5
Spellcasting: Clr7 (4th level spells), Wiz7 (4th level spells)
Obviously losing out on a few hp (and probably a few points on the fort save) haven't made this one sting very bad. The MyT is easily the most powerful of the multiclass patch prcs because of the double spellcasting.
Sor10
HP: 4+9d4 (avg 26.5)
BAB: +5
Spellcasting: Sor10 (5th level spells)
While these stats may not show it, this is the most powerful character out of all the ones listed. A Feeblemind would take any of the preceding out of the fight, the ability to teleport lets it fight on its own terms, a Wall of Force could trap either party with no way out, she can fly all day and turn any of the others into toads for her own amusement, and likely she has some permanent spells on her person. Her dispel magics have a high chance of dispelling buffs any of the others might have (at full +10 bonus), and against CRs of her level she adds a +10 (or more depending on feats) to her SR check.
Multiclassing is all about choices, but inherently you diversify yourself - diluting your strengths. When it comes to spellcasting you can dilute yourself into a corner, being so weak that you may not meaningfully affect combat at all (see the Clr5/Wiz5 for more info). These prcs patch the problem, that multiclassing with spellcasting classes (specifically wizard or sorceror) dilutes a character too much. Yes, its nice they can heal and cast fireballs, but what the party really needs is a teleport or a banishment, or for that fireball to actually hit the enemy, etc.
If this agrument doesn't even sway you, then you won't be swayed. Note that the above is assuming a face-less DM, one with no sympathy for his party. Certainly you can make allowances, any party CAN work, its all in the hands of the DMs, etc. Its just good to have a balanced system to begin with, which makes less work for all the dms.
Technik