Just a couple thoughts on the subject.
1) You're also giving up mobility in favour of relying on leg strength to controll your opponent.
2) It takes a long time to sit out or even perform a sweep from a body lock style closed guard and, especially when you are little like me, it's important to be able to stand up or go to your knees from guard to help negate your opponent's ability to use their weight.
3) Like Derek said though, it is very useful when using a high guard to lock out someone's shoulder. However, I would consider this the end part of a climb.
Hope this spurs further discussion.