So? RPGs are meant to be fun. If you are driving players to not do something because the system seems to not encourage it (or makes something appear "less fun,) then there is a flaw.
Thats not a flaw in the game though. Thats a flaw in perception.
If the game is too easy, its not fun. If there is no challenge, its not fun. Where is the challenge in having pat answers and powers for every situation all in one character? Sounds rather boring to me.
If C&C is supposed to be open and "play anything and do anything" then there is an inconsistent hypocrisy in the classes -- if there were not, there would be no need to have a "revise this and revise that" because people would look at the available options and think "wow, that does have a lot it can do" and not "well, under the right situations, with a Castle Keeper who interprets things this way, and if I were able to cast 8th level spells... then, yeah." Granted, that's always going to happen, but when it is occurring on a regular basis, the smart thing to do is to look and say "well, maybe there is something lacking..."
And if this were to be accomplished, it would take a rule book 10,000 pages long to cover every permutation that everyone can think of. Instead, folks can stop using rules in a book as a straight jacket and change things if they like.
Again, its a perceptual flaw. Not everyone points out the same flaws in the same classes consistently. Its all based on personal perception.
The illusionist needs a way to impact those which are otherwise immune, such as plants, oozes, most undead, constructs... they are effectively useless against an incredible range of monsters and situations.
Which can be said about a number of classes. The challenge then becomes making do with available resources or rely on the other characters to make up for the lack in a given class.
The illusionist can be just as effective against all those threats as any other class. Plants, oozes, undead are all flammable, and most characters carry a torch. And a construct is going to kick everyone's ass if they aren't quick enough.
"We'll go out through the kitchen!" Tanis Half-Elven
"The accumulated filth of all their sex and murder will foam up about their waists and all the whores and politicians will look up and shout, 'Save us!' And I'll look down, and whisper 'No.' " ~Rorschach