It's standard practice to use capping to protect the cable from getting nicked by the plasterers trowel, as you said, and is also fairly helpful in keeping the cable touching the wall to comply with method 'C'. If you're using capping then normally there's no point at all in clipping - in fact it can get in the way.

Do I have to cap electrical wires?

When you remove or open a wall or a ceiling, you may encounter stray electrical wires. ... But unless you intend to remove drywall up to the electric service panel, this usually isn't possible. The next-best approach is to terminate the electrical wires with wire caps (wire nuts).

Can you leave dead wires in a wall?

It's acceptable to leave wire in the walls. The only thing you need to do is leave the ends exposed in boxes and wire nut and tape the to legs together. That will indicate to an electrician what's going on, and if someone does try to tie into them in the future it will just pop the breaker.