Of course, I still feel that "feat bloat" or other forms of unchecked expansion of character customization options would be a far worse problem than "class bloat".
And I feel the opposite. Having a new class that necessitates a new character in order to use is of less value to me than a new feat which I could take next time I level up. (EDIT: Incidentally, I rarely plan out a build from beginning to end, and even when I do, I never follow it exactly. Rigid classes prevent organic growth without some serious flexibility in options. And, as I say repeatedly below, rigid /= flexible, and cannot, because they are exact opposites.)
If you have rigid classes, then you always know where to look up information you need during play.
And a rigid character, by definition, is inflexible. Sure the "Slayer" won't look like the "Archer," but every single "Archer" in a rigid class system will, by definition (again), look like every other archer. In a looser structure system any one "Fighter" will not look like any other "Fighter" because of the customization of a looser structure. Indeed, "Fighter" could be a "Slayer" or "Archer" simply from choosing different options within the structure. And, later, when the Fighter, who has previously chosen some "Slayer" options, decides to broaden his abilities, he isn't stuck with only the options in that a "Slayer" class would rigidly prevent.
If you have a large range of options taken from several different books, all relevant to your character, then it hurts mid-play usability. The two forms of "bloat" are simply not equivalent.
And I'd rather have more options in mid-play than simply doing the same thing over and over again.
The only way a "100 classes" system works, for me, is if I can build my own class* without ever being forced to use a "rigid" class that someone else designed. No other person's design goals are ever going to match up 100% with my own, and rigidity is at odds with flexibility, literally.
*Ideally, even if the 100 system model was used (and I have no reasons to suspect it would), it would ship with detailed instructions for how to create your own. Your "Fighter/Mage" may have more "Fighter" than what I'd want (maybe what I really want is a "Mage/Fighter"), which either necessitates a whole new class, or the tools for me to alter/build my own. And that means flexibility, not rigidity.