Rkhet said:
We are. Depending on the game length, it mightn't reach 20. Even if it does, it mightn't go into epic. So the earlier levels - in this case, level 15, weighs more than level 20.
Notice how in your progression, the difference in spell levels is higher for the earlier levels, and only catches up on level 20.
While it is true that the difference is more pronounced at level 15, it is hardly devastatingly weak.
Plus, most campaigns last for more than a single level. Or even three levels.
Even at level 15 where you are two levels behind, the spells are (assuming an 18 stat for one class ability score and a 19 in the other, where the 18 will become 19 at level 16):
06/07/07/07/6/5/4/3/2 15 Cleric
04/05/05/05/5/4/3/2/1 15 Wizard
05/06/06/06/6/5/4/3/2 15 Specialized Wizard
10/12/11/10/8/5/2/0/0 Cleric 3/Wizard 3/MysticTheurge 9
11/13/12/11/9/6/3/0/0 Cleric 3/Specialized Wizard 3/MysticTheurge 9
If you consider this to be a Cleric with some offensive spells, you are giving up at most 2 6th, 3 7th, and 2 8th level spells (which granted can be very potent) for 14 lower level spells and the ability to both spontaneously heal and cast offensive spells. Your caster level is the same if you take Practiced Spellcaster (and who wouldn't?).
Due to the lower BAB, hit dice, saves (except Will), and lack of armor, you really have to consider this character a Wizard who can heal himself (as opposed to a Cleric who can cast offensive spells since he cannot fill the role of a normal front line Cleric) which means you are giving up 4 high level spells (1 6th, 2 7th, and 1 8th) for 22 lower level ones and basically the ability to have virtually every spell that you might possibly need (Lesser Restoration, Neutralize Poison, Globe of Invulnerability, etc.) virtually every day. That amount of versatility is huge. You need all of your first level Wizard spell slots for other things, you put Protection From Evil into a Clerical first level spell slot.
I played a MT (Druid / Sorcerer) once and enjoyed it immensely just due the fact that I basically had a spell for nearly every situation.
And when prepped ahead of time, a divine / arcane character is defensively hard to beat. They have so many ways to protect and buff themselves it's scary. Your 15 Wizard does not have a Spell Resistance spell, no problem. The MT does. Your 15 Cleric does not have a Stoneskin spell, no problem. The MT does. Clerics cannot cast Cat's Grace, no problem. The MT can.