No part of the game rules say that to play you have to clear all content in 1 reset. 10mans are great for 25man guilds to provide something to do out of main raid hours, weekends, when everything is clear, and/or when you cant get 25 people online. In the depth of summer knowing that as a 25man guild you can do 25mans as and when possible and run 10mans for the rest of the time is a relief. The people who want to raid can, and the guild doesnt fall apart.