There is more to it than user-agent and screen size. There are tools out there like fraudfox that are intended to provide a way of masking the information. Last I was involved/interested it could reveal quite a bit of specific information. Typically, advanced fraudsters are capable of beating the systems. There are just too many tools available. Even blocking the trackers with noscript/privacy plug-ins can result in a block. Whatever comes out on the anti-fraud side is commonly just a temporary roadblock. The balance between convenience and security for most companies leans towards convenience.
