Not exactly a European example, but in Adelaide (South Australia) we currently have a ticketing system sourced from the French-based trans-national corporation Crouzet that uses validators.
They perform a number of tasks each time you validate...
The first is counting the number of people using the service, for revenue (private-operated buses use the same tickets as state-owned trains and trams) and capacity purposes. Even the free tickets given to over-65 passengers in the weekday inter-peak period need to be validated for this purpose so the required capacity can be determined.
The majority of our tickets are unlimited transfers within two hours (one trip) that come in singletrip and multitrips with ten trips on the same physical ticket. Validating for the first time starts that period with other validations in the next two hours counting on that trip, but after that time the next validation will count as the next trip if it's a multitrip, or a failed validation for a singletrip.
Exiting the platform area at Adelaide Station has ticket gates which use a quasi-validator which checks that it was validated on an incoming train. For this purpose the tickets count for an additional 45 minutes in case the valid period expires after transferring onto a train service
Validating when transferring to a different service allows revenue to be counted for all services used, not just the first one you get onto.
Once validated, a ticket is recorded as used. After the two hour trip period (singletrip), 10x cheaper trips (multitrip) or whole day of usage (daytrip) the ticket will fail to validate, which will draw the attention of the staff and/or block exit/entry at the Adelaide Station gates.
On trains and trams, the ticket can be checked by a hand-held machine to see if it has been validated to avoid people carrying the same ticket with them multiple times.
Like all ticketing systems, it has flaws (like the magnetic information getting wiped easily) but has remained reasonably serviceable for quite a while. It's not going away even with the new smart card system switched on in the testing phase, there having been no suitable alternatives to the singletrip or daytrip tickets identified for casual users.