Beware of 3 changing trends in regard of rail pass use:
- the number of trains requiring reservation (thus, fees for pass holders) is increasing. All but regional very slow, stop-everywhere services in Italy and Spain require reservation, for instance.
- Trenitalia and SNCF are aggressively introducing pass holder quotas. This mean that if you train to reserve a seat in a train with a pass, they will tell you there are not places, while one would be able to buy a ticket (without a pass) on the same train. Think of something akin to airlines limiting the availability of miles redemption passengers in each flight and all but blacklisting the most busy days of the year....
- joint-venture and niche companies like Thalys, Elypsos, Eurostar are racking up reservation fees, to the point it is usually cheaper to buy stand-alone advanced tickets Parix-Amsterdam or London-Bruxelles (for instance) with discount instead of paying hefty reservation fees. These companies all have pass holder quotas, particularly Thalys.
The fact is that with internet and the ready information about anything concerning tourist services like lodging, restaurant reservations etc., it is odd and even a bit outdated to want "complete flexibility" (that no longer exists) for rail transport. Complete flexibility is now the dominion of car drivers on vacation who can always decide what to do after breakfast.