Certifications by themselves hold little value. The value (and joy) in clearing certifications resides in the extra knowledge gained while preparing for them, brushing up basics and being able to use those skills and knowledge in the real world. Practice exams make one feel more confident and better prepared for the certification exam but the certification is just a means to an end and not the end itself.
Exam dumps are bad. I have seen quite a few people who memorize the typical questions and answers and manage to clear the exam. But needless to say these very people are usually unable to do most things expected of them in the real world.
This article on www.CIO.com - IT Certification Practice Exams Could Kill Your Career - talks about certification vendors cracking down on sites offering practice exams. It is in the vendors’ best interests to maintain the credibility of their certifications and one way of doing it is to ensure that dumps do not appear on the Internet. However another very solid business reason is to make prospective exam takers buy study material and practice exams from the vendors themselves or from other chosen sites having the vendors’ approval.
The mostly vacuous article mentioned above is forgetable till you come to the following gem - “Every time an individual takes a certification exam online, there are digital “fingerprints” that identify how long that person took to answer each question, whether he went back and changed any answers, and so on. Using data-forensics techniques, this digital evidence is analyzed for every exam taken. With incredible accuracy, the forensics reveal patterns that identify cheaters. Even inadvertent cheaters—those who didn’t know they used illegal preparatory materials—can be caught, but they are not distinguished from people who cheat intentionally.”
What a load of crap. This is nothing but very obvious FUD usage to further their aims. I actually found it very funny… 
