ITT, OP making theories about something they have never experienced.
Any legit heroic raiding guild has raiders who will do what they can to excel. This includes exhausting all options to get gear upgrades.
If there were no lockouts, many of these guilds would run the raid over, and over, and over to get BiS and push for progression. It would quickly become a competition to see not who the best players were, but who spent the most time farming in the first week. This would lead to burnout and like someone said above, lots of raiders would get that last boss kill and hang it up until the next patch.
On top of that, casual raiders would still be casual and probably wouldn't be running the extra content anyway.