Yo Doctor,
Let me alter your argument slightly.
"Dear Arcane Spellcaster,
Thank you for your interest in developing new areas of magical talent. You requested more information about options in the divine spellcasting arena, and we are pleased to tell you that divine spellcasting currently has many openings. We understand that you are currently 9th level. For your 10th character level, why not try cleric? With spells like Cure Light Wounds and Bless at your fingertips, you'll be adding new depth to your arsenal when fighting demons, devils, and young dragons of all colors!
Sincerely,
Board of Classes"
"Dear Board of Classes,
I received your letter, in which you advised me to consider a level in Cleric. However, this results in me diversifying and being really really weak. I mean, a Cure Light Wounds doesn't help me much as a 10th level character. Is there anything else we can do?
Yrs,
Ninth-level Arcane Spellcaster."
"Dear Spellcaster,
We understand the difficulty of your position. However, the rules are in place for a reason, and you were well aware of the consequences of trying to be as versatile as you wish to become. This kind of choice is really something you should have considered before. You do have options as a Mystic Theurge, of course.
Best of luck,
Board of Clases"
"Dear Board of Classes,
This is completely unfair. If I were a Fighter who wanted to make himself more versatile by adding levels of Paladin, Ranger, or Barbarian, I would have no disadvantages whatsoever. Please explain!"
"Dear Caster,
Well, the combat classes have always been overpowered."
There's nothing like roaming around on ENWorld and realizing that all the spellcasting classes are unfairly underpowered because they can't multiclass and stack their caster levels, while all the combat classes are underpowered because they don't get big incremental bumps like the casting classes get -- they just get that nice steady +1/level that just keeps stacking no matter which combat class they pick.
Man, all the classes are underpowered! These WotC folks botched it big!
Let me alter your argument slightly.

"Dear Arcane Spellcaster,
Thank you for your interest in developing new areas of magical talent. You requested more information about options in the divine spellcasting arena, and we are pleased to tell you that divine spellcasting currently has many openings. We understand that you are currently 9th level. For your 10th character level, why not try cleric? With spells like Cure Light Wounds and Bless at your fingertips, you'll be adding new depth to your arsenal when fighting demons, devils, and young dragons of all colors!
Sincerely,
Board of Classes"
"Dear Board of Classes,
I received your letter, in which you advised me to consider a level in Cleric. However, this results in me diversifying and being really really weak. I mean, a Cure Light Wounds doesn't help me much as a 10th level character. Is there anything else we can do?
Yrs,
Ninth-level Arcane Spellcaster."
"Dear Spellcaster,
We understand the difficulty of your position. However, the rules are in place for a reason, and you were well aware of the consequences of trying to be as versatile as you wish to become. This kind of choice is really something you should have considered before. You do have options as a Mystic Theurge, of course.
Best of luck,
Board of Clases"
"Dear Board of Classes,
This is completely unfair. If I were a Fighter who wanted to make himself more versatile by adding levels of Paladin, Ranger, or Barbarian, I would have no disadvantages whatsoever. Please explain!"
"Dear Caster,
Well, the combat classes have always been overpowered."
There's nothing like roaming around on ENWorld and realizing that all the spellcasting classes are unfairly underpowered because they can't multiclass and stack their caster levels, while all the combat classes are underpowered because they don't get big incremental bumps like the casting classes get -- they just get that nice steady +1/level that just keeps stacking no matter which combat class they pick.
Man, all the classes are underpowered! These WotC folks botched it big!