Goddess FallenAngel said:
I admit to being confused, because the method I described, which you quoted, has worked just fine for everything from 1st level to 26th. We've been using it in my gaming group since the book came out (I can never remember if it was in UA or AU - I get the names mixed up). Why does it not work for complex characters? I'd like to see an example.
Sure.

The first thing I noticed was that it was actually
much harder to do it this way, because you have to do a lot of subtraction and look at the difference between the current level and previous level. I'm actually not even sure I did it right because it was very confusing, so maybe you could check my work. (I also usually use the rule that you only get the +2 bonus for a good save once, not once for every new class, but I didn't use that with the method you described.)
1. Wiz1/War1 BAB +0, Fort +0, Ref +0, Will +2
2. Wiz2/War2 (simple incrementing)
3. Wiz3/War3 ...
4. Wiz4/War4 ...
5. Wiz5/War5 BAB +3, Fort +1, Ref +1, Will +4
6. Ult1/War6 BAB +4, Fort +2, Ref +2, Will +6
7. Swd1/Ult2 BAB +5, Fort +2, Ref +4, Will +8
8. Swd2/Ult3 BAB +6, Fort +3, Ref +5, Will +9
9. Ult4/War7 BAB +7, Fort +2, Ref +5, Will +10
10. Swd3/Ult5 BAB +8, Fort +3, Ref +5, Will +10
Compare that to my original version, which I consider "correct": BAB: +7, Fort +3, Ref +5, Will +7. Will is much higher because of multiclassing, BAB is slightly too high because of clever (or lucky) positioning of classes.
Here's the Wizard-Fighter/Wizard-Sorcerer build, using the "better of two values" method.
1 Wiz1/Ftr1 BAB +1 Fort +2 Ref +0 Will +2
2 Wiz2/Sor1 BAB +2 Fort +2 Ref +0 Will +5
3 Wiz3/Sor2 BAB +3 Fort +3 Ref +1 Will +5
4 Wiz4/Sor3 BAB +4 Fort +3 Ref +1 Will +6
...
20 Wiz20/Sor19 BAB +20 Fort +8 Ref +6 Will +14 (and caster levels of 20 and 19, respectively).
Compare that to my calculation of BAB +10, Fort +8, Ref +6, Will +12. Major difference in BAB. Obviously someone with 19 levels of Wizard-Sorcerer and one level of Wizard-Fighter shouldn't have a BAB higher than a regular Wiz19/Ftr1.