There is a nice page on the wiki about this introductory stuff and particularly about the use of the MVC acronym in Cat. Personally I agree with nothingmuch at #catalyst irc channel:

model should be the most complex reusable thing you can do without wasting time

For me it means that business logic should go into the model - but there are people that use some other definition of business logic. Generally it is usefull to reuse the model class in cron job or some other administrative scripts.