The price for a 30 day pass is not that much more than for a 15 day pass. And then the 60 day pass is not much more than a 30 day pass. Then the 90 day pass is very small up price compared to 60 day. So yeah the 90 day pass is a deal. But for a 15 day trip, I would not buy a pass at all - unless you are doing an insane trip like Paris to Vienna to Rome to Sicily ... and being on the train every day for a long trip.
As I mentioned, having the unlimited pass alters your travel plans. It makes since to take a 5 hour train from Munich to Hamburg, then stay 5 days there, and make a day trip to Bremen one day, then the opposite way to Lubeck the next day, etc. The first ticket might be 100 eu, and then the day trips might be 2 x 10 eu, or whatever.
In fact, in Alsace, we just stayed in a smaller town between Colmar and Strasbourg, and just rode the train up and down N or S to other towns - sometimes even twice a day - just commuting on trains that run every 30 minutes or so.
Buying each ticket then you choose more carefully - and maybe take a bus for a day trip
And with a car, you can stay in small outlying areas to save a ton of money and see a lot more - albeit with the danger of traffic tickets, accidents, damage, or getting lost.