So, am I missing a negative for the D-Ticket somehow?
No, it's a great deal for your airport trips, most local outings, and public transport within the major cities.
Twin Pass vs. Saver Fares: It might be about just those two trips, or it might not...
May I ask what outings you have in mind from Munich and from Berlin? Munich IMHO is not an especially good travel base for the typical outings tourists want to make from there as the train time often takes up 4 or more hours of the day by regional train. Most of these outings are only possible or preferably done by regional train, so the D-Ticket is the reasonable answer. But not always for some destinations... If Bamberg is a desired daytrip destination, it takes about 6 hours round trip on the D-Ticket. It would be perfectly reasonable to use the direct ICE train to Bamberg instead - 2 hours each way - and the Twin Pass would likely save you some cash in that case as well. The same goes for Stuttgart (Mercedes Benz Museum and more) which is just 2 hours from Munich by ICE but much much longer by regional train. Travel time to Nuremberg and back is feasible by regional train but can be cut roughly in half with the ICE option. Those are just a few examples. And the same might apply to your stay in Berlin... So if you have precise day trips such as the ones in the examples, you may be a good candidate for the 4- or 5-day twin pass (432 and 478 Euros respectively.)
The Twin Pass offers a bit of insurance... that is, you are protected against loss of the saver fare ticket value that you would forfeit if you were to miss one of your pre-scheduled saver fare trains, since you can ride ANY train you wish at any time on the travel day in question. It also allows you to travel on different days than scheduled/expected. If you wanted to stay someplace longer or leave it earlier than planned for whatever reason (health event for example) then you don't lose the saver fare value when you make that change, and you don't have to buy a new replacement ticket at a much higher price. Also, if you were to mistakenly board the wrong train with a saver fare (happens sometimes when there's a last-minute track change at the station) you will be subject to a penalty for riding the train without a valid ticket. Not so with the Twin Pass; you just hop off the wrong train and re-route yourselves on any trains that solve your problem.
The Twin Pass is valid not just for one journey but for a day of unlimited journeys. So it would be possible in your case to make a spontaneous stopover or detour to some other destination on your way from Munich to Berlin... Erfurt? Leipzig? We have often used the pass in this way... stowing bags in a locker for a few hours and have a long lunch, or do some targeted sightseeing, then get back on the train to our final destination whenever we are ready. This isn't possible at all with the saver fare ticket.
The Twin Pass, if you want one, can be bought at Munich airport or at Munich's main station once you arrive.
Hope this helps!
