If you are backpacking, then you're obviously trying to save money and do things as cheaply as possible. So the last thing you need or want is an expensive pass. France no longer accepts them; many other countries require seat reservations that can often cost more than point to point tickets. So your already expensive pass ends up costing you more and more as you use it.
Point to point tickets, bought well in advance will be the most cost effective way to travel. Of course, for the low prices you do sacrifice flexibility: your ticket is only good for the train, date and time that you booked.
Time now to sit down and work out a rough mud map of your itinerary. Then start looking at the national rail carriers for each leg of your journey. Buy from the national carrier at the point of departure; e.g. if going from Paris to Berlin, buy from the French SNFC (TVG Europe) site.
Tickets for most of Europe go on sale around 3 months before your travel date, and are cheapest at that time. They become progressively more expensive the closer you get to your travel dates. For Italy tickets go on sale 120 days before travel. For a few countries there may be no advantage in buying well in advance, but you can't work that out until you have some itinerary to work with.
Check out the man in seat 61 website: www.seat61.com
There's plenty of good information on passes, how and when to buy tickets, and cheapest options on there.
Edited: 03 February 2013, 8:57