The basic use of the hd is surely as a pedalboard for electric guitar.
Yes, I use it primarily for electric guitar and mainly for practice, rehearsals and playing on stage. We don't record that often.
We are doing some rough demo recordings for pre-production purposes to see how our songs can sound on CD and I'm thinking why not experiment to see if we can make them sound better. I will also be using the recordings for home practice.
I would prefer a good mic, a tube pre amp and a good reverb unit.
You can say exactly the same for guitar.
I don't see why you should not experiment and plug in a vocal mic., electric violin, keyboard or any other instrument after all it does have a mic. input and 'there are no rules' when it comes to creating music - apart of course how you rig it up, you don't want to blow up your equipment.
I know of violinists that have used it with good results and one violinist from another forum has emailed me his patches. I will try them when our violinist records her parts to see how they sound.