That is how it is for me, and those I played with. In Vanilla, I did the hardcore raiding, 4-5 days a week, doing whatever you need. TBC we switched to mostly 10 man, and partnered for some 25 man. WotLK and Cata we were pure 10 man, and only needed the odd dpps filler. During those two expansions, if we ever had more than 3people who couldn't make it, we called it. If ever I couldn't make it (I was gm, raid leader, and MT), we called it. Why? Because to me, we didn't hate ourselves.
Yes, if I wasn't there we could have found a pug tank, or have someone else bring an alt or off spec, and have someone else lead. But then you are dealing with people in different roles than normal, healing different, positioning slightly different, diffent leadership style. It is guaranteed not to be as efficient. And do all this for what? A chance at loot if thinks are on farm, or face smashing a progress boss for 3 hours. My guild and I had better things to do with our time. Sure, it meant it took us longer to progress. But we weren't in a race.
Once we went to this system, I would never go back.