Regarding the OP, I personally think that caster level = class level is too powerful. Especially for multiclassed characters who are also gaining class abilities, bonus feats, or whatever. I simply think that it is too powerful.
However, I also don't like the fractional values proposed in the other thread (or the varying rate, either). That adds complexity that I just don't like. People hate fractions enough as it is. Whole numbers and consistancyis the way to go for me. So, here's what I propose:
Caster level = character level - 1 per every three non-spellcaster class levels (round up).
So ... a rogue 1 / sorcerer 1 has a caster level of 1. A rogue 2 / sorcerer 1 has a caster level of 2. A rogue 3 / sorcerer 1 has a caster level of 3. A rogue 4 / sorcerer 1 has a caster level of 3. Etc.
The upside is that a theurge style now looks more playable without being over-the-top. A wizard 7 / cleric 3 / mystic theurge 10 has a wizard caster level of 19 and a cleric caster level of 17. Thus, not 20 /20 ... but 19 /17. I think that's a bit better, personally.
Additionally, I would argue that the response to non-caster multiclassing is a bit off. While I think a rogue 1 / fighter 5 is about the same amount of power as a fighter 6 or a rogue 6, I don't think that a rogue 3 / fighter 3 is as powerful as the full classed character. Ingeneral, I've found that the more balanced the levels are in multiclassing the worse the build is in terms of power. Just food for thought .... and only my opinion.
Now, for a bit of off-OP questioning ...
If one has a problem with multiclassing, why continue to use a classed system? Why not use a classless game? The one that got me hooked on classless design is Buy the Numbers. It is a quality product, but it didn't build characters using the progressions that I would have chosen. I've got a new product coming out through Dreamscarred Press that is also a classless design system.
Here is a link for the discussion thread. The product is in layout currently - due out soon.
Let me give you an example of what a classless system can do. Specifically, the classless system that I am publishing through DSP:
A "true" magus that has 20th level casting in both sorcerer (or wizard) and cleric. In one of those classes the caster has access to 8th level slots of this magnitude: 3/3/3/3/3/3/3/2/2. In the other class the character hass access to 9th level slots of this magnitude: 3/3/3/3/3/3/3/3/3/3. The character has 20d4, +10 BAB, typical saves for a 20th level caster, 7 total feats (IE, no bonus feats), and the cleric's ability to spontaneously cure. In essence, the character gains near full casting in both classes for a reduced amount of spell slots, the loss of turning, and what bonus feats the character might have gained.
I realize that what you're talking about isn't necessarily a true magus. You're talking about a caster class merging with something like rogue or fighter. But in a classless system you can do that and still have a caster level of 20. The diference between this and what you propose above is that you wouldn't automatically be able to gain all the class abilities, or HD, or BAB or whatever that you would get in standard multiclassing. There has to be a trade-off somewhere in order for a character to have caster ability = character level.