Chirag,
I think you have two use cases, that spring to mind. There may be others if you search the web.
1) Based on the application logic, the number of services invoked (and thus participants enlisted) could vary, dependent on input parameters. Therefore, sometimes you will only get one participant registered, making 2PC unnecessary. Imagine a travel agent, sometimes a customer will book a flight, hotel and taxi; other times, just a flight.
2) Even if multiple participants are enlisted, I think you can drive the final participant through 1PC if all previous participants voted "read-only".
I suggest you read the following book [1]. The Web services bit is out of date (describes old standards), however, the core concepts are still very much current. I always recommend that my Interns and any new team members read the book. I'd recommend reading chapter one (Transaction Fundamentals) and ten (Transactions and Web Services) in detail and then go through the other chapters in an order that interests you. It should give you a solid understanding of transaction concepts and will give you some good content for your thesis.
Our transactions blog is also a good source of information [2].
[1] http://www.amazon.co.uk/Java-Transaction-Processing-Design-Implementation/dp/013035290X
[2] http://jbossts.blogspot.co.uk/
Paul.