Hi Rizwan Mohammad,
Sorry for the late reply.
There are some other certifications related to XML but since IBM is one of the big companies who are the leaders in XML related technologies, people prefer to go for the IBM version.
I took the XML exam just because I wanted to gain more depth in the XML related area which, as most of us know is catching like a wild fire. Everything starting from configurations, XUL, Flex, VoiceXML,
SOAP and many more "hot" technologies use XML extensively. Truly speaking we dont need to read and type the XML since most of the tools do this automatically, understanding from behind the scenes will give you more confidence.
Long story short *
go for IBM XML exam* because it really helps.
Regarding the pre-assessment sample test, you dont need to buy them since there are quite good number of mock exams that you can find in the links page.
Cheers & Good luck.
Srikanth.