The value of isolation exercises

[Albert1]

Rip,
I see you include the barbell curl and tricep extensions in your SS book. In your experience and opinion, is there any value to these exercises or other isolation exercises at any point of anybody's training? Are they only good for bodybuilding trainers? Is there any value at all in any isolation exercise(s) for a trainee looking to improve his/her strength, fitness, health...etc? Or are all isolation exercises a waste of time?

[Mark Rippetoe]

Why do you think that barbell curls and tricep extensions, as described in BBT, are isolation exercises?