You and Uny both make very good points.  I guess it comes down to motivation.  In other words, why is someone using their number instead of their username?  I think it is abusive if they are doing so to make attacks and/or create trouble.  On the other hand, many times there may be understandable motives. 

For example, a private forum owner, known moderator or person with a fairly benevolent personna may wish to contribute and participate in the debate forums and keep to the high road but still not want to drag their known ID into it.  In other instances, a forum moderator may wish to contribute to the forum they moderate without it looking like the post came from a moderator and either giving it undue weight or else opening up the moderator to complaints of being biased or having an agenda - especially from those who haven't gotten their way or can't handle disagreement.

Sadly, as you and others have seen, there are some people who take offense at merely being disagreed with and who will not only refuse to let a debate drop but also harbor a grudge against those who refuse to agree with them, even to the point of stalking and threatening them.  One of the great downfalls of too many folks is simply knowing when to stop.  In some instances that may be due to an over-arching ego, in others it may be due to a lack of confidence in what they are saying.   In earlier days, I often found myself caught up in prolonged debates because I felt like I had to keep defending my position and carry the day.  Then I realized that if person feels strongly about being correct they should be able to make their case within just a few posts and not have to keep defending it - and let the readers make up their minds on the merits.  In most instances, over-debating probably erodes confidence from many readers and actually hurts your position more than helps it.  Not only that, but carrying on way past all reason may also erode people confidence and favorable impression of the poster overall and thus cause the good contributions they otherwise make to go unread.

It is a hard lesson to learn and goes against the nature of many who feel that they have to respond and defend themselves against every opposing message - and I speak from personal experience.  Now, I try to make my points thoroughly with fewer posts and then step away when others want to continue instead of rehashing the same points over and over - sometimes more successfully than others.  I suppose that old dogs can learn new tricks after all.  But some dogs are going to stay dogs no matter what (though I continue to hope that they will learn otherwise).

You are right in saying that few other forums posting as both a user name and as a user number.  However, I have seen several which allow you to post as either your username or as simply "anonymous".  Many don't allow it, but some certainly do.  To me, that can lead to even more abuse since there is no track history to try to follow - if that option is even available.

I guess that I am not sure which way I would go with the issue.  I will say that I often find it aggravating when I see people "hiding" behind a number.  I think that generally if you are going to put in your two cents you should step up to the plate and let people know where the two cents are coming from.  But I can see how others might disagree.

As for myself, I am pretty much an open book here, including having my actual name openly known and displayed. Because of that, I might be someone who would have good reasons to use a number ID at times.  But I don't.  Off the top of my head I would guess that I have posted as a number ID only two or three times out of about 10,000 posts over the years.