WildStar's design producer Stephan Frost saw him liken the subscription fee to the extra fee for viewing cable stations such as HBO. People pay the extra money every month because the channel produces good content, and a subscription MMO has to do the same thing to justify that ticket price.
Frost goes on to state that players should see new content in the game roughly every 28 days, with a vague mention that the team may do an expansion at some point but is mostly concerned with justifying the subscription price. It also ties into the CREDD system, which he states helps to cut down on gold-selling practices while also creating some parity between players with more real money and those with more real time. Time will see if the game justifies that subscription; you can try it out for yourself when the open beta goes live tomorrow.