Some time ago the affeuro program used to display 's tracking' as one of the benefits of their program, but I couldn't determine what that was (I'm unable to post attachments, but if you do a waybackmachine search and check records from 2010 and maybe also 2011 you should still see it).
Internet services like email (Yahoo) can track and record the actual serial number of a PC for security/privacy/fraud prevention purposes - to limit the number of accounts opened to 1 per user per computer. How do they know that more than one account is opened from a certain computer, they see the serial number of that PC at the time the account is opened. And if they see more than one account coming from the same PC they can assume that multiple accounts are being opened by a single user and close one of them.
...where there may be a problem is if the tracking of serial numbers is used as a bypass for 'cookies': if for example a recorded serial takes priority over the cookie placed on a person's PC, if a person comes to casino A from affiliate A and that person's PC is not on file at all, that 'serial' is assigned to that affiliate. If that same person using the same machine joins casino B from affiliate B, although there is a cookie, it may not be recorded for that affiliate - and the new account at casino B may be assigned to affiliate A based on his original connection to the serial. First come first served. It may even be that casinos take the financial risk of free signup bonuses for the purpose of harvesting as many computer serial numbers as possible, for the database.
...as long as aff programs are up-front about affiliates receiving/not receiving credit for their players switching between sister brands all is good. But if that detail is omitted and an incorrect representation made - and I'm not saying that's what affeurope did (I'm only referencing them for the example) - then there may be a problem...
Everyone's focusing on cookies and aff links being "dropped", but we may be overlooking an open barn door with this issue of serial tracking.