Eldritch_Lord
Adventurer
The MT has other balancing factors, like MAD stats and lower CL.
If you're going wizard/cleric, yes, and that was most likely the intended balance point. A wizard/archivist is Int SAD, and any combination with early entry is only a level or two behind and can easily get double 9s. A wizard 2/archivist 1/MT 17 has double 9s a single level behind a wizard 17 and has his pick of any spells he wants, which is reason enough not to extend MT by 4 levels.