The Luftwaffe was pretty good at using prototypes and "one-offs" even though they hadn't been accepted for production.
The majority of Ta154s did serve with NJG3, but they also made excellent high-speed trainers.
If memory serves right, they also used a few Ar240 aircraft as high-speed trainers for jet pilots, too. The Ar240 was intended to be a replacement for the Bf110 but lost the bid to the Me210...only 14 Ar240s were built and several were even used for high-speed recon.
In regards to the Ta154, it's a stroke of luck the Allies destroyed the Tego plant, as the substitute glue was problematic and caused a great deal trouble for the Luftwaffe and killing the Ta154 project.
Otherwise, the Ta154 would have meant serious trouble for the Allies.