From a Don Alessi Iron Dog column:
Q: Just why the heck are pull-ups superior to cable pulldowns?
A: The short answer? Because the biggest guys do them. The long answer? The pull-up forces you to train harder. Let's break it down.
The pull-up engages a greater number and higher threshold of motor units. This is due to the stabilizers firing to fixate the torso and keep it rigid and the relatively greater loads being placed on each muscle along the chain.
For example, it?s not uncommon to get serious DOMS (soreness) in the abs, glutes and hamstrings after eight sets of six reps of chins. With the pulldown, the lower and mid body is fixated by the machine and therefore those same muscles are inactive.
Additionally, many muscles along the chinning chain such as the abs, biceps, hamstrings and lower lat fibers are predominately fast twitch and respond best to the more intense effort that only the mighty pull-up can provide.
Let?s make one point clear. It?s only the "free," non-assisted pull-ups and chin-ups that are superior back developers. Assisted (machine) pull-ups are in many ways inferior to cable pulldowns. This is due to the potential for cheating on such devices. By flexing the hips, knees and or ankles on a pull-up machine, the trainee can create a significant plyometric effect. This leads to poor form and injury.
Stick to old fashioned pull-ups and chin-ups.
Now, although I don't gauge the effectiveness of a workout by the amount of soreness or DOMS I get, I do agree with Alessi.
Besides, as many Very Smart Buff Guys have said, the body functions as a unit in life and should be trained as one in workouts, at least most of the time.
Hopefully this answers your question. This was all I was able to find on the subject, but then it is almost 4am now, so I'm not really all here and only did a quick search. Do some reading on open-chain vs. closed-chain exercises for more thoughts.