Definition: This applies only to state tax returns. In community property states, married persons are considered to own their property, assets, and income jointly. If married people decide to file separate tax returns, they must follow their state rules for community income. Generally, both spouses must combine their total income, and divide that income in half. Each spouse then reports half of the joint income on his or her tax return. The community property states are:
Arizona
California
Idaho
Louisiana
Nevada
New Mexico
Texas
Washington
Wisconson

I copied the above from about.com

Community property also make is easy to handle property in the event of a illness or death of one of the partners by avoiding probate.

You also have to put a value on the the stay at home partner. It is very common for the stay at home partner that watchs pre-school kids to save more money then what they could make working wile the kid or kids are in daycare.