[Nachteule]

It is not necessary. It has been done on x86 system for years (running 32bit software/os on 64bit cpu)

But my understanding is that you will get more speed if you'll run native 64bit software

And you're right - getting more hardware and more power for the same price - why not. But its is not a RPi3, it is only a RPi2+
(but of course - RPi3 sounds much better)