The client should not store the user/pass [1]. If the token expires, the user needs to provide a user/pass to login in again. The user should also be forced to provide a user/pass in order to change the password - something that cannot be enforced if keeping the user/pass on the client.

You also lose any method to force a re-authentication. With a token, I could expire with no activity for an hour and allow it to be good for a max of 2 hours.

[1] Users have a bad habit of just leaving computers. With a token, the worst case is someone has a short lived access to to something. With a user/pass left on the client worst case now becomes use/pass taken.