Having given a witness statement in the past for a road traffic accident, and the subsequent messing around/days off work/trips to Magistrate's Court to give evidence to have the case adjourned twice, then returning for a third time to be told I was no longer required...I would say no, I probably wouldn't give a statement ever again!!
Sadly, I think that negative experiences like yours do have an effect on a persons willingness to put themselves forward again.
Other factors are involved as well, such as those mentioned by 47802, where it could be the case that the witness doesn't think the assault (or whatever) was serious enough to warrant their involvement, or that the victim was in some way responsible for what happened to them.
I'd like to think that I would give me details, but who knows what would happen if, for example, I saw a loud and aggressive person 'assaulted' by way of someone acting in self defence?
Yes, I know that would be for a court to decide, but I can't help making my own judgements!