I couldn't disagree more and not because I charge for coverage.
I don't pay for coverage, but I do pay for an professional editor to check over my spelling, grammar and typos, because those are my weak areas that require outside intervention.
If you have weak areas in structure, dialog, formatting, and you have read the books, read the produced scripts, watched the dvd's, and still have a weakness, then you need outside pro help.
For those people who do not have a network of readers with some experience (writer's groups, mentors, even here, etc...) having a pro read and critique your work is a valuable tool.
But with any tool, you must know when to use it and not abuse it.
Just because you did a great job putting those shelves up with that hammer, doesn't mean you will carry that hammer everywhere and use it for tuning your car, replacing a battery in your watch, etc...
Although everything Goldminer states is true, the true purpose of a pro reviewer is not to change your story around, but to make what you have now the strongest it can be with regards to structure, format and dialog.
A pro reviewer should give you a clear goal, if not, then I wouldn't use that person again (and I would even ask for your money back)...
Now of course, a script is never perfect and a producer will surely ask for more rewrites, even after the check clears, but a producer will not enjoy your script and see those glimmers of gold if it is chock full of formatting, structural and dialog problems.
So, if you are great with formatting, have a great sense of structure and always have outstanding dialog then you do not need pro coverage, but if not, then find one you like and use sparingly.
I would always email a pro reviewer first and ask what you get for your money. Their website may state it, but it's good to reaffirm. See if you can get a sample or an initial free consultation. shop around and don't pay thousands or even more than a few hundred.