Its for redundancy as you guessed but it is also for speed reasons, a database that is being heavily written to will be much slower than a database with heavy reads. And thus if you mix and match the two then the database will be slower, heavy locks are incurred with some select functions (count(*)) on innodb for instance....

Also, innodb is awesome for writing, but it doesn't allow the benefits that myisam does for reading (fulltext search etc). So it is more beneficial to use innodb as a slave as Adam suggests, and doing an INSERT INTO table SELECT * FROM write_table in order to get an up to date read table!

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